diff --git a/.eslintignore b/.eslintignore
index cc014c8b92f..5e8b92a1c90 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -1,7 +1,5 @@
 app/assets/javascripts/env.js
-app/assets/javascripts/main_include.js
 app/assets/javascripts/main_include_admin.js
-app/assets/javascripts/pagedown_custom.js
 app/assets/javascripts/vendor.js
 app/assets/javascripts/locales/i18n.js
 app/assets/javascripts/ember-addons/
@@ -11,11 +9,9 @@ lib/javascripts/messageformat.js
 lib/javascripts/moment.js
 lib/javascripts/moment_locale/
 lib/highlight_js/
+plugins/**/lib/javascripts/locale
 public/javascripts/
-spec/phantom_js/smoke_test.js
 vendor/
 test/javascripts/test_helper.js
-test/javascripts/test_helper.js
 test/javascripts/fixtures
 test/javascripts/helpers/assertions.js
-app/assets/javascripts/ember-addons/
diff --git a/app/assets/javascripts/locales/i18n.js b/app/assets/javascripts/locales/i18n.js
index ba56bc3a80c..b1efc2e1124 100644
--- a/app/assets/javascripts/locales/i18n.js
+++ b/app/assets/javascripts/locales/i18n.js
@@ -15,6 +15,7 @@ I18n.pluralizationRules = {
 
 // Set current locale to null
 I18n.locale = null;
+I18n.fallbackLocale = null;
 
 // Set the placeholder format. Accepts `{{placeholder}}` and `%{placeholder}`.
 I18n.PLACEHOLDER = /(?:\{\{|%\{)(.*?)(?:\}\}?)/gm;
@@ -143,6 +144,10 @@ I18n.translate = function(scope, options) {
   var translation = this.lookup(scope, options);
 
   if (!this.noFallbacks) {
+    if (!translation && this.fallbackLocale) {
+      options.locale = this.fallbackLocale;
+      translation = this.lookup(scope, options);
+    }
     if (!translation && this.currentLocale() !== this.defaultLocale) {
       options.locale = this.defaultLocale;
       translation = this.lookup(scope, options);
diff --git a/app/models/locale_site_setting.rb b/app/models/locale_site_setting.rb
index b38b5f85ac1..ae3c02d3ccd 100644
--- a/app/models/locale_site_setting.rb
+++ b/app/models/locale_site_setting.rb
@@ -7,9 +7,12 @@ class LocaleSiteSetting < EnumSiteSetting
   end
 
   def self.values
-    supported_locales.map do |l|
-      lang = language_names[l] || language_names[l[0..1]]
-      { name: lang ? lang['nativeName'] : l, value: l }
+    @values ||= supported_locales.map do |locale|
+      lang = language_names[locale] || language_names[locale.split("_")[0]]
+      {
+        name: lang ? lang['nativeName'] : locale,
+        value: locale
+      }
     end
   end
 
@@ -19,43 +22,41 @@ class LocaleSiteSetting < EnumSiteSetting
     return @language_names if @language_names
 
     @lock.synchronize do
-      @language_names ||= YAML.load(File.read(File.join(Rails.root, 'config', 'locales', 'names.yml')))
+      @language_names ||= begin
+        names = YAML.load(File.read(File.join(Rails.root, 'config', 'locales', 'names.yml')))
+
+        DiscoursePluginRegistry.locales.each do |locale, options|
+          if !names.key?(locale) && options[:name] && options[:nativeName]
+            names[locale] = { "name" => options[:name], "nativeName" => options[:nativeName] }
+          end
+        end
+
+        names
+      end
     end
   end
 
   def self.supported_locales
     @lock.synchronize do
       @supported_locales ||= begin
-        app_client_files = Dir.glob(
+        locales = Dir.glob(
           File.join(Rails.root, 'config', 'locales', 'client.*.yml')
-        )
+        ).map { |x| x.split('.')[-2] }
 
-        unless ignore_plugins?
-          app_client_files += Dir.glob(
-            File.join(Rails.root, 'plugins', '*', 'config', 'locales', 'client.*.yml')
-          )
-        end
-
-        app_client_files.map { |x| x.split('.')[-2] }
-          .uniq
-          .select { |locale| valid_locale?(locale) }
-          .sort
+        locales += DiscoursePluginRegistry.locales.keys
+        locales.uniq.sort
       end
     end
   end
 
-  def self.valid_locale?(locale)
-    assets = Rails.configuration.assets
-
-    assets.precompile.grep(/locales\/#{locale}(?:\.js)?/).present? &&
-      (Dir.glob(File.join(Rails.root, 'app', 'assets', 'javascripts', 'locales', "#{locale}.js.erb")).present? ||
-        Dir.glob(File.join(Rails.root, 'plugins', '*', 'assets', 'locales', "#{locale}.js.erb")).present?)
+  def self.reset!
+    @lock.synchronize do
+      @values = @language_names = @supported_locales = nil
+    end
   end
 
-  def self.ignore_plugins?
-    Rails.env.test? && ENV['LOAD_PLUGINS'] != "1"
+  def self.fallback_locale(locale)
+    plugin_locale = DiscoursePluginRegistry.locales[locale.to_s]
+    plugin_locale ? plugin_locale[:fallbackLocale]&.to_sym : nil
   end
-
-  private_class_method :valid_locale?
-  private_class_method :ignore_plugins?
 end
diff --git a/app/models/translation_override.rb b/app/models/translation_override.rb
index 4c2ac9b82ca..ea3859930c5 100644
--- a/app/models/translation_override.rb
+++ b/app/models/translation_override.rb
@@ -21,7 +21,8 @@ class TranslationOverride < ActiveRecord::Base
 
     data = { value: value }
     if key.end_with?('_MF')
-      data[:compiled_js] = JsLocaleHelper.compile_message_format(locale, value)
+      _, filename = JsLocaleHelper.find_message_format_locale(['en'], false)
+      data[:compiled_js] = JsLocaleHelper.compile_message_format(filename, locale, value)
     end
 
     translation_override = find_or_initialize_by(params)
diff --git a/config/locales/names.yml b/config/locales/names.yml
index 6f15ab632d6..60e9925014a 100644
--- a/config/locales/names.yml
+++ b/config/locales/names.yml
@@ -119,9 +119,6 @@ eo:
 es:
   name: Spanish
   nativeName: Español
-es_MX:
-  name: Spanish
-  nativeName: Español (MX)
 et:
   name: Estonian
   nativeName: eesti
diff --git a/lib/discourse_plugin_registry.rb b/lib/discourse_plugin_registry.rb
index afe8009f8c5..e46cf891fd8 100644
--- a/lib/discourse_plugin_registry.rb
+++ b/lib/discourse_plugin_registry.rb
@@ -14,6 +14,7 @@ class DiscoursePluginRegistry
     attr_writer :handlebars
     attr_writer :serialized_current_user_fields
     attr_writer :seed_data
+    attr_writer :locales
     attr_accessor :custom_html
 
     def plugins
@@ -65,6 +66,10 @@ class DiscoursePluginRegistry
       @seed_data ||= HashWithIndifferentAccess.new({})
     end
 
+    def locales
+      @locales ||= HashWithIndifferentAccess.new({})
+    end
+
     def html_builders
       @html_builders ||= {}
     end
@@ -92,6 +97,10 @@ class DiscoursePluginRegistry
     self.class.stylesheets << filename
   end
 
+  def self.register_locale(locale, options = {})
+    self.locales[locale] = options
+  end
+
   def register_archetype(name, options = {})
     Archetype.register(name, options)
   end
@@ -171,6 +180,10 @@ class DiscoursePluginRegistry
     result.uniq
   end
 
+  def locales
+    self.class.locales
+  end
+
   def javascripts
     self.class.javascripts
   end
@@ -207,6 +220,7 @@ class DiscoursePluginRegistry
     self.desktop_stylesheets = nil
     self.sass_variables = nil
     self.handlebars = nil
+    self.locales = nil
   end
 
   def self.reset!
@@ -222,6 +236,7 @@ class DiscoursePluginRegistry
     html_builders.clear
     vendored_pretty_text.clear
     seed_path_builders.clear
+    locales.clear
   end
 
   def self.setup(plugin_class)
diff --git a/lib/freedom_patches/translate_accelerator.rb b/lib/freedom_patches/translate_accelerator.rb
index 92f0b80b311..fe737f52578 100644
--- a/lib/freedom_patches/translate_accelerator.rb
+++ b/lib/freedom_patches/translate_accelerator.rb
@@ -39,6 +39,13 @@ module I18n
         if @loaded_locales.empty?
           # load all rb files
           I18n.backend.load_translations(I18n.load_path.grep(/\.rb$/))
+
+          # load plural rules from plugins
+          DiscoursePluginRegistry.locales.each do |locale, options|
+            if options[:plural]
+              I18n.backend.store_translations(locale, i18n: { plural: options[:plural] })
+            end
+          end
         end
 
         # load it
diff --git a/lib/i18n/backend/fallback_locale_list.rb b/lib/i18n/backend/fallback_locale_list.rb
index 29d9700dbaa..6b4faf1bf1b 100644
--- a/lib/i18n/backend/fallback_locale_list.rb
+++ b/lib/i18n/backend/fallback_locale_list.rb
@@ -3,7 +3,8 @@ module I18n
     # Configure custom fallback order
     class FallbackLocaleList < Hash
       def [](locale)
-        [locale, SiteSetting.default_locale.to_sym, :en].uniq.compact
+        fallback_locale = LocaleSiteSetting.fallback_locale(locale)
+        [locale, fallback_locale, SiteSetting.default_locale.to_sym, :en].uniq.compact
       end
     end
   end
diff --git a/lib/js_locale_helper.rb b/lib/js_locale_helper.rb
index 59a9c27c3cb..af8d5fbfacc 100644
--- a/lib/js_locale_helper.rb
+++ b/lib/js_locale_helper.rb
@@ -85,6 +85,7 @@ module JsLocaleHelper
   end
 
   def self.load_translations_merged(*locales)
+    locales = locales.compact
     @loaded_merges ||= {}
     @loaded_merges[locales.join('-')] ||= begin
       all_translations = {}
@@ -101,9 +102,10 @@ module JsLocaleHelper
   end
 
   def self.translations_for(locale_str)
-    current_locale = I18n.locale
-    locale_sym     = locale_str.to_sym
-    site_locale    = SiteSetting.default_locale.to_sym
+    current_locale  = I18n.locale
+    locale_sym      = locale_str.to_sym
+    site_locale     = SiteSetting.default_locale.to_sym
+    fallback_locale = LocaleSiteSetting.fallback_locale(locale_str)
 
     I18n.locale = locale_sym
 
@@ -113,9 +115,9 @@ module JsLocaleHelper
       elsif locale_sym == :en
         load_translations(locale_sym)
       elsif locale_sym == site_locale || site_locale == :en
-        load_translations_merged(locale_sym, :en)
+        load_translations_merged(locale_sym, fallback_locale, :en)
       else
-        load_translations_merged(locale_sym, site_locale, :en)
+        load_translations_merged(locale_sym, fallback_locale, site_locale, :en)
       end
 
     I18n.locale = current_locale
@@ -125,11 +127,13 @@ module JsLocaleHelper
 
   def self.output_locale(locale)
     locale_str = locale.to_s
+    fallback_locale_str = LocaleSiteSetting.fallback_locale(locale_str)&.to_s
     translations = Marshal.load(Marshal.dump(translations_for(locale_str)))
 
     message_formats = strip_out_message_formats!(translations[locale_str]['js'])
     message_formats.merge!(strip_out_message_formats!(translations[locale_str]['admin_js']))
-    result = generate_message_format(message_formats, locale_str)
+    mf_locale, mf_filename = find_message_format_locale([locale_str], true)
+    result = generate_message_format(message_formats, mf_locale, mf_filename)
 
     translations.keys.each do |l|
       translations[l].keys.each do |k|
@@ -140,7 +144,8 @@ module JsLocaleHelper
     # I18n
     result << "I18n.translations = #{translations.to_json};\n"
     result << "I18n.locale = '#{locale_str}';\n"
-    result << "I18n.pluralizationRules.#{locale_str} = MessageFormat.locale.#{locale_str};\n" if locale_str != "en"
+    result << "I18n.fallbackLocale = '#{fallback_locale_str}';\n" if fallback_locale_str && fallback_locale_str != "en"
+    result << "I18n.pluralizationRules.#{locale_str} = MessageFormat.locale.#{mf_locale};\n" if mf_locale != "en"
 
     # moment
     result << File.read("#{Rails.root}/lib/javascripts/moment.js")
@@ -150,6 +155,41 @@ module JsLocaleHelper
     result
   end
 
+  def self.find_moment_locale(locale_chain)
+    path = "#{Rails.root}/lib/javascripts/moment_locale"
+
+    # moment.js uses a different naming scheme for locale files
+    locale_chain = locale_chain.map { |l| l.tr('_', '-').downcase }
+
+    find_locale(locale_chain, path, :moment_js, false)
+  end
+
+  def self.find_message_format_locale(locale_chain, fallback_to_english)
+    path = "#{Rails.root}/lib/javascripts/locale"
+    find_locale(locale_chain, path, :message_format, fallback_to_english)
+  end
+
+  def self.find_locale(locale_chain, path, type, fallback_to_english)
+    locale_chain.each do |locale|
+      plugin_locale = DiscoursePluginRegistry.locales[locale]
+      return plugin_locale[type] if plugin_locale&.has_key?(type)
+
+      filename = File.join(path, "#{locale}.js")
+      return [locale, filename] if File.exist?(filename)
+    end
+
+    # try again, but this time only with the language itself
+    locale_chain = locale_chain.map { |l| l.split(/[-_]/)[0] }
+      .uniq.reject { |l| locale_chain.include?(l) }
+    unless locale_chain.empty?
+      locale_data = find_locale(locale_chain, path, type, false)
+      return locale_data if locale_data
+    end
+
+    # English should alyways work
+    ["en", File.join(path, "en.js")] if fallback_to_english
+  end
+
   def self.moment_formats
     result = ""
     result << moment_format_function('short_date_no_year')
@@ -163,23 +203,13 @@ module JsLocaleHelper
     "moment.fn.#{name.camelize(:lower)} = function(){ return this.format('#{format}'); };\n"
   end
 
-  def self.moment_locale(locale_str)
-    # moment.js uses a different naming scheme for locale files
-    locale_str = locale_str.tr('_', '-').downcase
-    filename = "#{Rails.root}/lib/javascripts/moment_locale/#{locale_str}.js"
-
-    # try the language without the territory
-    locale_str = locale_str.split("-")[0]
-    filename = "#{Rails.root}/lib/javascripts/moment_locale/#{locale_str}.js" unless File.exists?(filename)
-
-    File.exists?(filename) ? File.read(filename) << "\n" : ""
+  def self.moment_locale(locale)
+    _, filename = find_moment_locale([locale])
+    filename && File.exist?(filename) ? File.read(filename) << "\n" : ""
   end
 
-  def self.generate_message_format(message_formats, locale_str)
-    formats = message_formats.map { |k, v| k.inspect << " : " << compile_message_format(locale_str, v) }.join(", ")
-
-    filename = "#{Rails.root}/lib/javascripts/locale/#{locale_str}.js"
-    filename = "#{Rails.root}/lib/javascripts/locale/en.js" unless File.exists?(filename)
+  def self.generate_message_format(message_formats, locale, filename)
+    formats = message_formats.map { |k, v| k.inspect << " : " << compile_message_format(filename, locale, v) }.join(", ")
 
     result = "MessageFormat = {locale: {}};\n"
     result << "I18n._compiledMFs = {#{formats}};\n"
@@ -203,10 +233,9 @@ module JsLocaleHelper
     end
   end
 
-  def self.compile_message_format(locale, format)
+  def self.compile_message_format(path, locale, format)
     with_context do |ctx|
-      path = "#{Rails.root}/lib/javascripts/locale/#{locale}.js"
-      ctx.load(path) if File.exists?(path)
+      ctx.load(path) if File.exist?(path)
       ctx.eval("mf = new MessageFormat('#{locale}');")
       ctx.eval("mf.precompile(mf.parse(#{format.inspect}))")
     end
diff --git a/lib/plugin/instance.rb b/lib/plugin/instance.rb
index 197606da79b..91953a10747 100644
--- a/lib/plugin/instance.rb
+++ b/lib/plugin/instance.rb
@@ -29,6 +29,7 @@ class Plugin::Instance
    :color_schemes,
    :initializers,
    :javascripts,
+   :locales,
    :service_workers,
    :styles,
    :themes].each do |att|
@@ -319,6 +320,14 @@ class Plugin::Instance
     javascripts << js
   end
 
+  # @option opts [String] :name
+  # @option opts [String] :nativeName
+  # @option opts [String] :fallbackLocale
+  # @option opts [Hash] :plural
+  def register_locale(locale, opts = {})
+    locales << [locale, opts]
+  end
+
   def register_custom_html(hash)
     DiscoursePluginRegistry.custom_html ||= {}
     DiscoursePluginRegistry.custom_html.merge!(hash)
@@ -427,7 +436,7 @@ JS
     end
 
     register_assets! unless assets.blank?
-
+    register_locales!
     register_service_workers!
 
     seed_data.each do |key, value|
@@ -532,6 +541,33 @@ JS
     end
   end
 
+  def register_locales!
+    root_path = File.dirname(@path)
+
+    locales.each do |locale, opts|
+      opts = opts.dup
+      opts[:client_locale_file] = File.join(root_path, "config/locales/client.#{locale}.yml")
+      opts[:server_locale_file] = File.join(root_path, "config/locales/server.#{locale}.yml")
+      opts[:js_locale_file] = File.join(root_path, "assets/locales/#{locale}.js.erb")
+
+      locale_chain = opts[:fallbackLocale] ? [locale, opts[:fallbackLocale]] : [locale]
+      lib_locale_path = File.join(root_path, "lib/javascripts/locale")
+
+      path = File.join(lib_locale_path, "message_format")
+      opts[:message_format] = find_locale_file(locale_chain, path)
+      opts[:message_format] = JsLocaleHelper.find_message_format_locale(locale_chain, false) unless opts[:message_format]
+
+      path = File.join(lib_locale_path, "moment_js")
+      opts[:moment_js] = find_locale_file(locale_chain, path)
+      opts[:moment_js] = JsLocaleHelper.find_moment_locale(locale_chain) unless opts[:moment_js]
+
+      if valid_locale?(opts)
+        DiscoursePluginRegistry.register_locale(locale, opts)
+        Rails.configuration.assets.precompile << "locales/#{locale}.js"
+      end
+    end
+  end
+
   private
 
   def write_asset(path, contents)
@@ -553,4 +589,18 @@ JS
     yield plugin
   end
 
+  def valid_locale?(custom_locale)
+    File.exist?(custom_locale[:client_locale_file]) &&
+      File.exist?(custom_locale[:server_locale_file]) &&
+      File.exist?(custom_locale[:js_locale_file]) &&
+      custom_locale[:message_format] && custom_locale[:moment_js]
+  end
+
+  def find_locale_file(locale_chain, path)
+    locale_chain.each do |locale|
+      filename = File.join(path, "#{locale}.js")
+      return [locale, filename] if File.exist?(filename)
+    end
+    nil
+  end
 end
diff --git a/spec/components/fallback_locale_list_spec.rb b/spec/components/fallback_locale_list_spec.rb
new file mode 100644
index 00000000000..2786ff1cd6f
--- /dev/null
+++ b/spec/components/fallback_locale_list_spec.rb
@@ -0,0 +1,48 @@
+require 'rails_helper'
+require 'i18n/backend/fallback_locale_list'
+
+describe I18n::Backend::FallbackLocaleList do
+  let(:list) { I18n::Backend::FallbackLocaleList.new }
+
+  it "works when default_locale is English" do
+    SiteSetting.default_locale = :en
+
+    expect(list[:ru]).to eq([:ru, :en])
+    expect(list[:en]).to eq([:en])
+  end
+
+  it "works when default_locale is not English" do
+    SiteSetting.default_locale = :de
+
+    expect(list[:ru]).to eq([:ru, :de, :en])
+    expect(list[:de]).to eq([:de, :en])
+    expect(list[:en]).to eq([:en, :de])
+  end
+
+  context "when plugin registered fallback locale" do
+    before do
+      DiscoursePluginRegistry.register_locale("es_MX", fallbackLocale: "es")
+      DiscoursePluginRegistry.register_locale("de_AT", fallbackLocale: "de")
+    end
+
+    after do
+      DiscoursePluginRegistry.reset!
+    end
+
+    it "works when default_locale is English" do
+      SiteSetting.default_locale = :en
+
+      expect(list[:de_AT]).to eq([:de_AT, :de, :en])
+      expect(list[:de]).to eq([:de, :en])
+      expect(list[:en]).to eq([:en])
+    end
+
+    it "works when default_locale is not English" do
+      SiteSetting.default_locale = :de
+
+      expect(list[:es_MX]).to eq([:es_MX, :es, :de, :en])
+      expect(list[:es]).to eq([:es, :de, :en])
+      expect(list[:en]).to eq([:en, :de])
+    end
+  end
+end
diff --git a/spec/components/freedom_patches/translate_accelerator_spec.rb b/spec/components/freedom_patches/translate_accelerator_spec.rb
index 883edd6f1c4..941b68e52a1 100644
--- a/spec/components/freedom_patches/translate_accelerator_spec.rb
+++ b/spec/components/freedom_patches/translate_accelerator_spec.rb
@@ -1,6 +1,9 @@
 require "rails_helper"
 
 describe "translate accelerator" do
+  after do
+    I18n.reload!
+  end
 
   it "overrides for both string and symbol keys" do
     key = "user.email.not_allowed"
@@ -32,4 +35,39 @@ describe "translate accelerator" do
     end
   end
 
+  context "plugins" do
+    before do
+      DiscoursePluginRegistry.register_locale(
+        "foo",
+        name: "Foo",
+        nativeName: "Foo Bar",
+        plural: {
+          keys: [:one, :few, :other],
+          rule: lambda do |n|
+            return :one if n == 1
+            return :few if n < 10
+            :other
+          end
+        }
+      )
+
+      LocaleSiteSetting.reset!
+      I18n.reload!
+    end
+
+    after do
+      DiscoursePluginRegistry.reset!
+      LocaleSiteSetting.reset!
+    end
+
+    it "loads plural rules from plugins" do
+      I18n.backend.store_translations(:foo, items: { one: 'one item', few: 'some items', other: "%{count} items" })
+      I18n.locale = :foo
+
+      expect(I18n.t('i18n.plural.keys')).to eq([:one, :few, :other])
+      expect(I18n.t('items', count: 1)).to eq('one item')
+      expect(I18n.t('items', count: 3)).to eq('some items')
+      expect(I18n.t('items', count: 20)).to eq('20 items')
+    end
+  end
 end
diff --git a/spec/components/js_locale_helper_spec.rb b/spec/components/js_locale_helper_spec.rb
index 120b6b029bd..430a372504c 100644
--- a/spec/components/js_locale_helper_spec.rb
+++ b/spec/components/js_locale_helper_spec.rb
@@ -34,12 +34,17 @@ describe JsLocaleHelper do
   end
 
   context "message format" do
+    def message_format_filename(locale)
+      Rails.root + "lib/javascripts/locale/#{locale}.js"
+    end
 
     def setup_message_format(format)
+      filename = message_format_filename('en')
+      compiled = JsLocaleHelper.compile_message_format(filename, 'en', format)
+
       @ctx = MiniRacer::Context.new
       @ctx.eval('MessageFormat = {locale: {}};')
-      @ctx.load(Rails.root + 'lib/javascripts/locale/en.js')
-      compiled = JsLocaleHelper.compile_message_format('en', format)
+      @ctx.load(filename)
       @ctx.eval("var test = #{compiled}")
     end
 
@@ -110,7 +115,7 @@ describe JsLocaleHelper do
     end
 
     it 'load pluralizations rules before precompile' do
-      message = JsLocaleHelper.compile_message_format('ru', 'format')
+      message = JsLocaleHelper.compile_message_format(message_format_filename('ru'), 'ru', 'format')
       expect(message).not_to match 'Plural Function not found'
     end
   end
diff --git a/spec/components/plugin/instance_spec.rb b/spec/components/plugin/instance_spec.rb
index 7c1f37f6a50..82e9cb3a2bd 100644
--- a/spec/components/plugin/instance_spec.rb
+++ b/spec/components/plugin/instance_spec.rb
@@ -10,8 +10,8 @@ describe Plugin::Instance do
   context "find_all" do
     it "can find plugins correctly" do
       plugins = Plugin::Instance.find_all("#{Rails.root}/spec/fixtures/plugins")
-      expect(plugins.count).to eq(1)
-      plugin = plugins[0]
+      expect(plugins.count).to eq(2)
+      plugin = plugins[1]
 
       expect(plugin.name).to eq("plugin-name")
       expect(plugin.path).to eq("#{Rails.root}/spec/fixtures/plugins/my_plugin/plugin.rb")
@@ -268,4 +268,108 @@ describe Plugin::Instance do
       expect(called).to eq(1)
     end
   end
+
+  context "locales" do
+    let(:plugin_path) { "#{Rails.root}/spec/fixtures/plugins/custom_locales" }
+    let!(:plugin) { Plugin::Instance.new(nil, "#{plugin_path}/plugin.rb") }
+    let(:plural) do
+      {
+        keys: [:one, :few, :other],
+        rule: lambda do |n|
+          return :one if n == 1
+          return :few if n < 10
+          :other
+        end
+      }
+    end
+
+    def register_locale(locale, opts)
+      plugin.register_locale(locale, opts)
+      plugin.activate!
+
+      DiscoursePluginRegistry.locales[locale]
+    end
+
+    it "enables the registered locales only on activate" do
+      plugin.register_locale("foo", name: "Foo", nativeName: "Foo Bar", plural: plural)
+      plugin.register_locale("es_MX", name: "Spanish (Mexico)", nativeName: "Español (México)", fallbackLocale: "es")
+      expect(DiscoursePluginRegistry.locales.count).to eq(0)
+
+      plugin.activate!
+      expect(DiscoursePluginRegistry.locales.count).to eq(2)
+    end
+
+    it "allows finding the locale by string and symbol" do
+      register_locale("foo", name: "Foo", nativeName: "Foo Bar", plural: plural)
+
+      expect(DiscoursePluginRegistry.locales).to have_key(:foo)
+      expect(DiscoursePluginRegistry.locales).to have_key('foo')
+    end
+
+    it "correctly registers a new locale" do
+      locale = register_locale("foo", name: "Foo", nativeName: "Foo Bar", plural: plural)
+
+      expect(DiscoursePluginRegistry.locales.count).to eq(1)
+      expect(DiscoursePluginRegistry.locales).to have_key(:foo)
+
+      expect(locale[:fallbackLocale]).to be_nil
+      expect(locale[:message_format]).to eq(["foo", "#{plugin_path}/lib/javascripts/locale/message_format/foo.js"])
+      expect(locale[:moment_js]).to eq(["foo", "#{plugin_path}/lib/javascripts/locale/moment_js/foo.js"])
+      expect(locale[:plural]).to eq(plural.with_indifferent_access)
+
+      expect(Rails.configuration.assets.precompile).to include("locales/foo.js")
+    end
+
+    it "correctly registers a new locale using a fallback locale" do
+      locale = register_locale("es_MX", name: "Spanish (Mexico)", nativeName: "Español (México)", fallbackLocale: "es")
+
+      expect(DiscoursePluginRegistry.locales.count).to eq(1)
+      expect(DiscoursePluginRegistry.locales).to have_key(:es_MX)
+
+      expect(locale[:fallbackLocale]).to eq("es")
+      expect(locale[:message_format]).to eq(["es", "#{Rails.root}/lib/javascripts/locale/es.js"])
+      expect(locale[:moment_js]).to eq(["es", "#{Rails.root}/lib/javascripts/moment_locale/es.js"])
+      expect(locale[:plural]).to be_nil
+
+      expect(Rails.configuration.assets.precompile).to include("locales/es_MX.js")
+    end
+
+    it "correctly registers a new locale when some files exist in core" do
+      locale = register_locale("tlh", name: "Klingon", nativeName: "tlhIngan Hol", plural: plural)
+
+      expect(DiscoursePluginRegistry.locales.count).to eq(1)
+      expect(DiscoursePluginRegistry.locales).to have_key(:tlh)
+
+      expect(locale[:fallbackLocale]).to be_nil
+      expect(locale[:message_format]).to eq(["tlh", "#{plugin_path}/lib/javascripts/locale/message_format/tlh.js"])
+      expect(locale[:moment_js]).to eq(["tlh", "#{Rails.root}/lib/javascripts/moment_locale/tlh.js"])
+      expect(locale[:plural]).to eq(plural.with_indifferent_access)
+
+      expect(Rails.configuration.assets.precompile).to include("locales/tlh.js")
+    end
+
+    it "does not register a new locale when the fallback locale does not exist" do
+      register_locale("bar", name: "Bar", nativeName: "Bar", fallbackLocale: "foo")
+      expect(DiscoursePluginRegistry.locales.count).to eq(0)
+    end
+
+    [
+      "config/locales/client.foo.yml",
+      "config/locales/server.foo.yml",
+      "lib/javascripts/locale/message_format/foo.js",
+      "lib/javascripts/locale/moment_js/foo.js",
+      "assets/locales/foo.js.erb"
+    ].each do |path|
+      it "does not register a new locale when #{path} is missing" do
+        path = "#{plugin_path}/#{path}"
+        File.stubs('exist?').returns(false)
+        File.stubs('exist?').with(regexp_matches(/#{Regexp.quote(plugin_path)}.*/)).returns(true)
+        File.stubs('exist?').with(path).returns(false)
+
+        register_locale("foo", name: "Foo", nativeName: "Foo Bar", plural: plural)
+        expect(DiscoursePluginRegistry.locales.count).to eq(0)
+      end
+    end
+  end
+
 end
diff --git a/spec/fixtures/plugins/custom_locales/assets/locales/es_MX.js.erb b/spec/fixtures/plugins/custom_locales/assets/locales/es_MX.js.erb
new file mode 100644
index 00000000000..d07d575a09e
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/assets/locales/es_MX.js.erb
@@ -0,0 +1,2 @@
+//= require locales/i18n
+<%= JsLocaleHelper.output_locale(:es_MX) %>
diff --git a/spec/fixtures/plugins/custom_locales/assets/locales/foo.js.erb b/spec/fixtures/plugins/custom_locales/assets/locales/foo.js.erb
new file mode 100644
index 00000000000..601dadb7774
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/assets/locales/foo.js.erb
@@ -0,0 +1,2 @@
+//= require locales/i18n
+<%= JsLocaleHelper.output_locale(:foo) %>
diff --git a/spec/fixtures/plugins/custom_locales/assets/locales/tlh.js.erb b/spec/fixtures/plugins/custom_locales/assets/locales/tlh.js.erb
new file mode 100644
index 00000000000..2e3284dc996
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/assets/locales/tlh.js.erb
@@ -0,0 +1,2 @@
+//= require locales/i18n
+<%= JsLocaleHelper.output_locale(:tlh) %>
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/client.es_MX.yml b/spec/fixtures/plugins/custom_locales/config/locales/client.es_MX.yml
new file mode 100644
index 00000000000..c9cb23e960b
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/client.es_MX.yml
@@ -0,0 +1 @@
+es_MX:
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/client.foo.yml b/spec/fixtures/plugins/custom_locales/config/locales/client.foo.yml
new file mode 100644
index 00000000000..0152a79652d
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/client.foo.yml
@@ -0,0 +1 @@
+foo:
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/client.tlh.yml b/spec/fixtures/plugins/custom_locales/config/locales/client.tlh.yml
new file mode 100644
index 00000000000..884714fb718
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/client.tlh.yml
@@ -0,0 +1 @@
+tlh:
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/server.es_MX.yml b/spec/fixtures/plugins/custom_locales/config/locales/server.es_MX.yml
new file mode 100644
index 00000000000..c9cb23e960b
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/server.es_MX.yml
@@ -0,0 +1 @@
+es_MX:
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/server.foo.yml b/spec/fixtures/plugins/custom_locales/config/locales/server.foo.yml
new file mode 100644
index 00000000000..0152a79652d
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/server.foo.yml
@@ -0,0 +1 @@
+foo:
diff --git a/spec/fixtures/plugins/custom_locales/config/locales/server.tlh.yml b/spec/fixtures/plugins/custom_locales/config/locales/server.tlh.yml
new file mode 100644
index 00000000000..884714fb718
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/config/locales/server.tlh.yml
@@ -0,0 +1 @@
+tlh:
diff --git a/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/foo.js b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/foo.js
new file mode 100644
index 00000000000..79b527258a0
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/foo.js
@@ -0,0 +1 @@
+// this file should contain plural rules
diff --git a/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/tlh.js b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/tlh.js
new file mode 100644
index 00000000000..79b527258a0
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/message_format/tlh.js
@@ -0,0 +1 @@
+// this file should contain plural rules
diff --git a/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/moment_js/foo.js b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/moment_js/foo.js
new file mode 100644
index 00000000000..971b3b1a1ca
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/lib/javascripts/locale/moment_js/foo.js
@@ -0,0 +1 @@
+// this file should contain the locale configuration for moment.js
diff --git a/spec/fixtures/plugins/custom_locales/plugin.rb b/spec/fixtures/plugins/custom_locales/plugin.rb
new file mode 100644
index 00000000000..700c68cb5f8
--- /dev/null
+++ b/spec/fixtures/plugins/custom_locales/plugin.rb
@@ -0,0 +1,4 @@
+# name: custom-locales
+# about: Fixtures for plugin that adds new locales
+# version: 1.0
+# authors: Gerhard Schlager
diff --git a/spec/models/locale_site_setting_spec.rb b/spec/models/locale_site_setting_spec.rb
index ef81765dc07..06406385eb1 100644
--- a/spec/models/locale_site_setting_spec.rb
+++ b/spec/models/locale_site_setting_spec.rb
@@ -1,6 +1,15 @@
 require 'rails_helper'
 
 describe LocaleSiteSetting do
+  def core_locales
+    pattern = File.join(Rails.root, 'config', 'locales', 'client.*.yml')
+    Dir.glob(pattern).map { |x| x.split('.')[-2] }
+  end
+
+  def native_locale_name(locale)
+    value = LocaleSiteSetting.values.find { |v| v[:value] == locale }
+    value[:name]
+  end
 
   describe 'valid_value?' do
     it 'returns true for a locale that we have translations for' do
@@ -14,8 +23,69 @@ describe LocaleSiteSetting do
 
   describe 'values' do
     it 'returns all the locales that we have translations for' do
-      expect(LocaleSiteSetting.values.map { |x| x[:value] }).to include(*Dir.glob(File.join(Rails.root, 'config', 'locales', 'client.*.yml')).map { |x| x.split('.')[-2] })
+      expect(LocaleSiteSetting.values.map { |x| x[:value] }).to include(*core_locales)
+    end
+
+    it 'returns native names' do
+      expect(native_locale_name('de')).to eq('Deutsch')
+      expect(native_locale_name('zh_CN')).to eq('中文')
+      expect(native_locale_name('zh_TW')).to eq('中文 (TW)')
     end
   end
 
+  context 'with locales from plugin' do
+    before do
+      DiscoursePluginRegistry.register_locale("foo", name: "Foo", nativeName: "Native Foo")
+      DiscoursePluginRegistry.register_locale("bar", name: "Bar", nativeName: "Native Bar")
+      DiscoursePluginRegistry.register_locale("de", name: "Renamed German", nativeName: "Native renamed German")
+      DiscoursePluginRegistry.register_locale("de_AT", name: "German (Austria)", nativeName: "Österreichisch", fallbackLocale: "de")
+      DiscoursePluginRegistry.register_locale("tlh")
+
+      # Plugins normally register a locale before LocaleSiteSetting is initialized.
+      # That's not happening in tests, so we need to call reset!
+      LocaleSiteSetting.reset!
+    end
+
+    after do
+      DiscoursePluginRegistry.reset!
+    end
+
+    describe 'valid_value?' do
+      it 'returns true for locales from core' do
+        expect(LocaleSiteSetting.valid_value?('en')).to eq(true)
+        expect(LocaleSiteSetting.valid_value?('de')).to eq(true)
+      end
+
+      it 'returns true for locales added by plugins' do
+        expect(LocaleSiteSetting.valid_value?('foo')).to eq(true)
+        expect(LocaleSiteSetting.valid_value?('bar')).to eq(true)
+      end
+    end
+
+    describe 'values' do
+      it 'returns native names added by plugin' do
+        expect(native_locale_name('foo')).to eq('Native Foo')
+        expect(native_locale_name('bar')).to eq('Native Bar')
+      end
+
+      it 'does not allow plugins to override native names that exist in core' do
+        expect(native_locale_name('de')).to eq('Deutsch')
+      end
+
+      it 'returns the language code when no nativeName is set' do
+        expect(native_locale_name('tlh')).to eq('tlh')
+      end
+    end
+
+    describe 'fallback_locale' do
+      it 'returns the fallback locale registered by plugin' do
+        expect(LocaleSiteSetting.fallback_locale('de_AT')).to eq(:de)
+        expect(LocaleSiteSetting.fallback_locale(:de_AT)).to eq(:de)
+      end
+
+      it 'returns nothing when no fallback locale was registered' do
+        expect(LocaleSiteSetting.fallback_locale('foo')).to be_nil
+      end
+    end
+  end
 end
diff --git a/test/javascripts/lib/i18n-test.js.es6 b/test/javascripts/lib/i18n-test.js.es6
index 314c4cc6938..e653a8d0ca3 100644
--- a/test/javascripts/lib/i18n-test.js.es6
+++ b/test/javascripts/lib/i18n-test.js.es6
@@ -1,17 +1,30 @@
 QUnit.module("lib:i18n", {
   _locale: I18n.locale,
+  _fallbackLocale: I18n.fallbackLocale,
   _translations: I18n.translations,
 
   beforeEach() {
     I18n.locale = "fr";
 
     I18n.translations = {
+      "fr_FOO": {
+        "js": {
+          "topic": {
+            "reply": {
+              "title": "Foo"
+            }
+          },
+        }
+      },
       "fr": {
         "js": {
           "hello": "Bonjour",
           "topic": {
             "reply": {
-              "title": "Répondre",
+              "title": "Répondre"
+            },
+            "share": {
+              "title": "Partager"
             }
           },
           "character_count": {
@@ -56,6 +69,7 @@ QUnit.module("lib:i18n", {
 
   afterEach() {
     I18n.locale = this._locale;
+    I18n.fallbackLocale = this._fallbackLocale;
     I18n.translations = this._translations;
   }
 });
@@ -92,4 +106,13 @@ QUnit.test("pluralizations", assert => {
   assert.equal(I18n.t("word_count", { count: 3 }), "3 words");
   assert.equal(I18n.t("word_count", { count: 10 }), "10 words");
   assert.equal(I18n.t("word_count", { count: 100 }), "100 words");
-});
\ No newline at end of file
+});
+
+QUnit.test("fallback", assert => {
+  I18n.locale = "fr_FOO";
+  I18n.fallbackLocale = "fr";
+
+  assert.equal(I18n.t("topic.reply.title"), "Foo", "uses locale translations when they exist");
+  assert.equal(I18n.t("topic.share.title"), "Partager", "falls back to fallbackLocale translations when they exist");
+  assert.equal(I18n.t("topic.reply.help"), "begin composing a reply to this topic", "falls back to English translations");
+});