FEATURE: Localization fallbacks (client)

This patch sets I18n.defaultLocale in the Discourse.start() script block (it
was formerly always 'en') to SiteSetting.default_locale, and patches
translate() to perform fallback to defaultLocale followed by english.
Additionally, when enable_verbose_localization() is called, no fallbacks will
be performed.

It also memoizes the file loading operations in JsLocaleHelper and strips out
translations from the fallbacks that are also present in a prefered language,
to minimize file size.

app/assets/javascripts/locales/i18n.js

@@ -52,6 +52,8 @@ I18n.PLACEHOLDER = /(?:\{\{|%\{)(.*?)(?:\}\}?)/gm;
 
 I18n.fallbackRules = {};
 
+I18n.noFallbacks = false;
+
 I18n.pluralizationRules = {
   en: function(n) {
     return n === 0 ? ["zero", "none", "other"] : n === 1 ? "one" : "other";
@@ -192,6 +194,15 @@ I18n.interpolate = function(message, options) {
 I18n.translate = function(scope, options) {
   options = this.prepareOptions(options);
   var translation = this.lookup(scope, options);
+  // Fallback to the default locale
+  if (!translation && this.currentLocale() !== this.defaultLocale && !this.noFallbacks) {
+    options.locale = this.defaultLocale;
+    translation = this.lookup(scope, options);
+  }
+  if (!translation && this.currentLocale() !== 'en' && !this.noFallbacks) {
+    options.locale = 'en';
+    translation = this.lookup(scope, options);
+  }
 
   try {
     if (typeof translation === "object") {
@@ -513,6 +524,7 @@ I18n.enable_verbose_localization = function(){
   var keys = {};
   var t = I18n.t;
 
+  I18n.noFallbacks = true;
 
   I18n.t = I18n.translate = function(scope, value){
     var current = keys[scope];

app/views/common/_discourse_javascript.html.erb

@@ -33,6 +33,7 @@
   Discourse.Environment = '<%= Rails.env %>';
   Discourse.SiteSettings = PreloadStore.get('siteSettings');
   Discourse.LetterAvatarVersion = '<%= LetterAvatar.version %>';
+  I18n.defaultLocale = '<%= SiteSetting.default_locale %>';
   PreloadStore.get("customEmoji").forEach(function(emoji) {
     Discourse.Dialect.registerEmoji(emoji.name, emoji.url);
   });

lib/js_locale_helper.rb

@@ -1,13 +1,12 @@
 module JsLocaleHelper
 
-  def self.output_locale(locale, translations = nil)
+  def self.load_translations(locale)
-    current_locale = I18n.locale
+    @loaded_translations ||= {}
-    I18n.locale = locale.to_sym
+    @loaded_translations[locale] ||= begin
-
       locale_str = locale.to_s
 
       # load default translations
-    translations ||= YAML::load(File.open("#{Rails.root}/config/locales/client.#{locale_str}.yml"))
+      translations = YAML::load(File.open("#{Rails.root}/config/locales/client.#{locale_str}.yml"))
       # load plugins translations
       plugin_translations = {}
       Dir["#{Rails.root}/plugins/*/config/locales/client.#{locale_str}.yml"].each do |file|
@@ -25,6 +24,66 @@ module JsLocaleHelper
       admin_contents = translations[locale_str].delete('admin_js')
       translations[locale_str]['js'].deep_merge!(admin_contents) if admin_contents.present?
       translations[locale_str]['js'].deep_merge!(plugin_translations[locale_str]['admin_js']) if translations[locale_str] && plugin_translations[locale_str] && plugin_translations[locale_str]['admin_js']
+
+      translations
+    end
+  end
+
+  # purpose-built recursive algorithm ahoy!
+  def self.deep_delete_matches(deleting_from, *checking_hashes)
+    checking_hashes.compact!
+
+    new_hash = deleting_from.dup
+    deleting_from.each do |key, value|
+      if value.is_a? Hash
+        # Recurse
+        new_at_key = deep_delete_matches(deleting_from[key], *(checking_hashes.map {|h| h[key]}))
+        if new_at_key.empty?
+          new_hash.delete key
+        else
+          new_hash[key] = new_at_key
+        end
+      else
+        if checking_hashes.any? {|h| h.include? key}
+          new_hash.delete key
+        end
+      end
+    end
+    new_hash
+  end
+
+  def self.load_translations_merged(*locales)
+    @loaded_merges ||= {}
+    @loaded_merges[locales.join('-')] ||= begin
+      # TODO - this will need to be reworked to support N fallbacks in the future
+      all_translations = locales.map { |l| JsLocaleHelper.load_translations l }
+      merged_translations = {}
+      merged_translations[locales[0].to_s] = all_translations[0][locales[0].to_s]
+      if locales[1]
+        merged_translations[locales[1].to_s] = deep_delete_matches(all_translations[1][locales[1].to_s].dup, merged_translations[locales[0].to_s])
+      end
+      if locales[2]
+        merged_translations[locales[2].to_s] = deep_delete_matches(all_translations[2][locales[2].to_s].dup, merged_translations[locales[0].to_s], merged_translations[locales[1].to_s])
+      end
+      merged_translations
+    end
+  end
+
+  def self.output_locale(locale, request=nil)
+    current_locale = I18n.locale
+    I18n.locale = locale.to_sym
+
+    locale_str = locale.to_s
+    site_locale = SiteSetting.default_locale.to_sym
+
+    if locale == :en
+      translations = load_translations(locale)
+    elsif locale == site_locale || site_locale == :en
+      translations = load_translations_merged(locale, :en)
+    else
+      translations = load_translations_merged(locale, site_locale, :en)
+    end
+
     message_formats = strip_out_message_formats!(translations[locale_str]['js'])
 
     result = generate_message_format(message_formats, locale_str)