diff --git a/app/assets/javascripts/discourse/app/initializers/localization.js b/app/assets/javascripts/discourse/app/initializers/localization.js
index 0c0657075c3..25d14c7864e 100644
--- a/app/assets/javascripts/discourse/app/initializers/localization.js
+++ b/app/assets/javascripts/discourse/app/initializers/localization.js
@@ -24,35 +24,28 @@ export default {
     }
 
     // Merge any overrides into our object
-    const overrides = I18n._overrides || {};
-    Object.keys(overrides).forEach((k) => {
-      const v = overrides[k];
-      k = k.replace("admin_js", "js");
+    for (const [locale, overrides] of Object.entries(I18n._overrides || {})) {
+      for (const [key, value] of Object.entries(overrides)) {
+        const segs = key.replace(/^admin_js\./, "js.admin.").split(".");
+        let node = I18n.translations[locale] || {};
 
-      const segs = k.split(".");
-
-      let node = I18n.translations[I18n.locale] || {};
-      let i = 0;
-
-      for (; i < segs.length - 1; i++) {
-        if (!(segs[i] in node)) {
-          node[segs[i]] = {};
+        for (let i = 0; i < segs.length - 1; i++) {
+          if (!(segs[i] in node)) {
+            node[segs[i]] = {};
+          }
+          node = node[segs[i]];
+        }
+
+        if (typeof node === "object") {
+          node[segs[segs.length - 1]] = value;
         }
-        node = node[segs[i]];
       }
+    }
 
-      if (typeof node === "object") {
-        node[segs[segs.length - 1]] = v;
-      }
-    });
-
-    const mfOverrides = I18n._mfOverrides || {};
-    Object.keys(mfOverrides).forEach((k) => {
-      const v = mfOverrides[k];
-
-      k = k.replace(/^[a-z_]*js\./, "");
-      I18n._compiledMFs[k] = v;
-    });
+    for (let [key, value] of Object.entries(I18n._mfOverrides || {})) {
+      key = key.replace(/^[a-z_]*js\./, "");
+      I18n._compiledMFs[key] = value;
+    }
 
     bootbox.addLocale(I18n.currentLocale(), {
       OK: I18n.t("composer.modal_ok"),
diff --git a/app/assets/javascripts/discourse/tests/unit/localization-test.js b/app/assets/javascripts/discourse/tests/unit/localization-test.js
index 6d8f303bbef..0c2cf9ba80e 100644
--- a/app/assets/javascripts/discourse/tests/unit/localization-test.js
+++ b/app/assets/javascripts/discourse/tests/unit/localization-test.js
@@ -6,7 +6,10 @@ import { getApplication } from "@ember/test-helpers";
 module("initializer:localization", {
   _locale: I18n.locale,
   _translations: I18n.translations,
+  _extras: I18n.extras,
+  _compiledMFs: I18n._compiledMFs,
   _overrides: I18n._overrides,
+  _mfOverrides: I18n._mfOverrides,
 
   beforeEach() {
     I18n.locale = "fr";
@@ -15,16 +18,43 @@ module("initializer:localization", {
       fr: {
         js: {
           composer: {
-            reply: "Répondre",
+            both_languages1: "composer.both_languages1 (FR)",
+            both_languages2: "composer.both_languages2 (FR)",
           },
         },
       },
       en: {
         js: {
-          topic: {
-            reply: {
-              help: "begin composing a reply to this topic",
-            },
+          composer: {
+            both_languages1: "composer.both_languages1 (EN)",
+            both_languages2: "composer.both_languages2 (EN)",
+            only_english1: "composer.only_english1 (EN)",
+            only_english2: "composer.only_english2 (EN)",
+          },
+        },
+      },
+    };
+
+    I18n._compiledMFs = {
+      "user.messages.some_key_MF": () => "user.messages.some_key_MF (FR)",
+    };
+
+    I18n.extras = {
+      fr: {
+        admin: {
+          api: {
+            both_languages1: "admin.api.both_languages1 (FR)",
+            both_languages2: "admin.api.both_languages2 (FR)",
+          },
+        },
+      },
+      en: {
+        admin: {
+          api: {
+            both_languages1: "admin.api.both_languages1 (EN)",
+            both_languages2: "admin.api.both_languages2 (EN)",
+            only_english1: "admin.api.only_english1 (EN)",
+            only_english2: "admin.api.only_english2 (EN)",
           },
         },
       },
@@ -34,35 +64,115 @@ module("initializer:localization", {
   afterEach() {
     I18n.locale = this._locale;
     I18n.translations = this._translations;
+    I18n.extras = this._extras;
+    I18n._compiledMFs = this._compiledMFs;
     I18n._overrides = this._overrides;
+    I18n._mfOverrides = this._mfOverrides;
   },
 });
 
 test("translation overrides", function (assert) {
   I18n._overrides = {
-    "js.composer.reply": "WAT",
-    "js.topic.reply.help": "foobar",
+    fr: {
+      "js.composer.both_languages1": "composer.both_languages1 (FR override)",
+      "js.composer.only_english2": "composer.only_english2 (FR override)",
+    },
+    en: {
+      "js.composer.both_languages2": "composer.both_languages2 (EN override)",
+      "js.composer.only_english1": "composer.only_english1 (EN override)",
+    },
   };
   LocalizationInitializer.initialize(getApplication());
 
   assert.strictEqual(
-    I18n.t("composer.reply"),
-    "WAT",
+    I18n.t("composer.both_languages1"),
+    "composer.both_languages1 (FR override)",
     "overrides existing translation in current locale"
   );
+
   assert.strictEqual(
-    I18n.t("topic.reply.help"),
-    "foobar",
-    "overrides translation in default locale"
+    I18n.t("composer.only_english1"),
+    "composer.only_english1 (EN override)",
+    "overrides translation in fallback locale"
+  );
+
+  assert.strictEqual(
+    I18n.t("composer.only_english2"),
+    "composer.only_english2 (FR override)",
+    "overrides translation that doesn't exist in current locale"
+  );
+
+  assert.strictEqual(
+    I18n.t("composer.both_languages2"),
+    "composer.both_languages2 (FR)",
+    "prefers translation in current locale over override in fallback locale"
+  );
+});
+
+test("translation overrides (admin_js)", function (assert) {
+  I18n._overrides = {
+    fr: {
+      "admin_js.api.both_languages1": "admin.api.both_languages1 (FR override)",
+      "admin_js.api.only_english2": "admin.api.only_english2 (FR override)",
+    },
+    en: {
+      "admin_js.api.both_languages2": "admin.api.both_languages2 (EN override)",
+      "admin_js.api.only_english1": "admin.api.only_english1 (EN override)",
+    },
+  };
+  LocalizationInitializer.initialize(getApplication());
+
+  assert.strictEqual(
+    I18n.t("admin.api.both_languages1"),
+    "admin.api.both_languages1 (FR override)",
+    "overrides existing translation in current locale"
+  );
+
+  assert.strictEqual(
+    I18n.t("admin.api.only_english1"),
+    "admin.api.only_english1 (EN override)",
+    "overrides translation in fallback locale"
+  );
+
+  assert.strictEqual(
+    I18n.t("admin.api.only_english2"),
+    "admin.api.only_english2 (FR override)",
+    "overrides translation that doesn't exist in current locale"
+  );
+
+  assert.strictEqual(
+    I18n.t("admin.api.both_languages2"),
+    "admin.api.both_languages2 (FR)",
+    "prefers translation in current locale over override in fallback locale"
+  );
+});
+
+test("translation overrides for MessageFormat strings", function (assert) {
+  I18n._mfOverrides = {
+    "js.user.messages.some_key_MF": () =>
+      "user.messages.some_key_MF (FR override)",
+  };
+
+  LocalizationInitializer.initialize(getApplication());
+
+  assert.strictEqual(
+    I18n.messageFormat("user.messages.some_key_MF", {}),
+    "user.messages.some_key_MF (FR override)",
+    "overrides existing MessageFormat string"
   );
 });
 
 test("skip translation override if parent node is not an object", function (assert) {
   I18n._overrides = {
-    "js.composer.reply": "WAT",
-    "js.composer.reply.help": "foobar",
+    fr: {
+      "js.composer.both_languages1.foo":
+        "composer.both_languages1.foo (FR override)",
+    },
   };
   LocalizationInitializer.initialize(getApplication());
 
-  assert.strictEqual(I18n.t("composer.reply.help"), "[fr.composer.reply.help]");
+  assert.strictEqual(
+    I18n.t("composer.both_languages1.foo"),
+    "[fr.composer.both_languages1.foo]"
+  );
 });
diff --git a/app/controllers/admin/site_texts_controller.rb b/app/controllers/admin/site_texts_controller.rb
index 55e3611f1dd..a3ed9f7d607 100644
--- a/app/controllers/admin/site_texts_controller.rb
+++ b/app/controllers/admin/site_texts_controller.rb
@@ -183,7 +183,7 @@ class Admin::SiteTextsController < Admin::AdminController
   def find_translations(query, overridden, locale)
     translations = Hash.new { |hash, key| hash[key] = {} }
     search_results = I18n.with_locale(locale) do
-      I18n.search(query, overridden: overridden)
+      I18n.search(query, only_overridden: overridden)
     end
 
     search_results.each do |key, value|
diff --git a/lib/js_locale_helper.rb b/lib/js_locale_helper.rb
index e12fab50150..4ca3cd5864a 100644
--- a/lib/js_locale_helper.rb
+++ b/lib/js_locale_helper.rb
@@ -160,31 +160,44 @@ module JsLocaleHelper
     result
   end
 
-  def self.output_client_overrides(locale)
-    overrides = TranslationOverride
-      .where(locale: locale)
-      .where("translation_key LIKE 'js.%' OR translation_key LIKE 'admin_js.%'")
-      .pluck(:translation_key, :value, :compiled_js)
+  def self.output_client_overrides(main_locale)
+    all_overrides = {}
+    has_overrides = false
 
-    return "" if overrides.blank?
+    I18n.fallbacks[main_locale].each do |locale|
+      overrides = all_overrides[locale] = TranslationOverride
+        .where(locale: locale)
+        .where("translation_key LIKE 'js.%' OR translation_key LIKE 'admin_js.%'")
+        .pluck(:translation_key, :value, :compiled_js)
 
-    message_formats = []
-    translations = {}
-
-    overrides.each do |key, value, compiled_js|
-      if key.end_with?("_MF")
-        message_formats << "#{key.inspect}: #{compiled_js}"
-      else
-        translations[key] = value
-      end
+      has_overrides ||= overrides.present?
     end
 
-    message_formats = message_formats.join(", ")
+    return "" if !has_overrides
 
-    <<~JS
-      I18n._mfOverrides = {#{message_formats}};
-      I18n._overrides = #{translations.to_json};
-    JS
+    result = +"I18n._overrides = {};"
+    existing_keys = Set.new
+    message_formats = []
+
+    all_overrides.each do |locale, overrides|
+      translations = {}
+
+      overrides.each do |key, value, compiled_js|
+        next if existing_keys.include?(key)
+        existing_keys << key
+
+        if key.end_with?("_MF")
+          message_formats << "#{key.inspect}: #{compiled_js}"
+        else
+          translations[key] = value
+        end
+      end
+
+      result << "I18n._overrides['#{locale}'] = #{translations.to_json};" if translations.present?
+    end
+
+    result << "I18n._mfOverrides = {#{message_formats.join(", ")}};"
+    result
   end
 
   def self.output_extra_locales(bundle, locale)
diff --git a/spec/requests/admin/site_texts_controller_spec.rb b/spec/requests/admin/site_texts_controller_spec.rb
index 90a0bd546f1..01ccc442d0c 100644
--- a/spec/requests/admin/site_texts_controller_spec.rb
+++ b/spec/requests/admin/site_texts_controller_spec.rb
@@ -153,6 +153,19 @@ RSpec.describe Admin::SiteTextsController do
         expect(value).to eq('education.new-topic override')
       end
 
+      it "returns only overridden translations" do
+        TranslationOverride.upsert!(:en, 'education.new-topic', 'education.new-topic override')
+
+        get "/admin/customize/site_texts.json", params: { locale: 'en', overridden: true }
+        expect(response.status).to eq(200)
+
+        site_texts = response.parsed_body['site_texts']
+        expect(site_texts.size).to eq(1)
+
+        value = site_texts.find { |text| text['id'] == 'education.new-topic' }['value']
+        expect(value).to eq('education.new-topic override')
+      end
+
       context 'plural keys' do
         before do
           I18n.backend.store_translations(:en, colour: { one: '%{count} colour', other: '%{count} colours' })
diff --git a/spec/requests/extra_locales_controller_spec.rb b/spec/requests/extra_locales_controller_spec.rb
index 4fad83de519..371e2c4f6bb 100644
--- a/spec/requests/extra_locales_controller_spec.rb
+++ b/spec/requests/extra_locales_controller_spec.rb
@@ -109,10 +109,39 @@ describe ExtraLocalesController do
           ctx.eval("I18n = {};")
           ctx.eval(response.body)
 
-          expect(ctx.eval('typeof I18n._mfOverrides["js.client_MF"]')).to eq("function")
-          expect(ctx.eval('I18n._overrides["js.some_key"]')).to eq("client-side translation")
-          expect(ctx.eval('I18n._overrides["js.client_MF"] === undefined')).to eq(true)
-          expect(ctx.eval('I18n._overrides["admin_js.another_key"]')).to eq("admin client js")
+          expect(ctx.eval("typeof I18n._mfOverrides['js.client_MF']")).to eq("function")
+          expect(ctx.eval("I18n._overrides['#{I18n.locale}']['js.some_key']")).to eq("client-side translation")
+          expect(ctx.eval("I18n._overrides['#{I18n.locale}']['js.client_MF'] === undefined")).to eq(true)
+          expect(ctx.eval("I18n._overrides['#{I18n.locale}']['admin_js.another_key']")).to eq("admin client js")
+        end
+
+        it "returns overrides from fallback locale" do
+          TranslationOverride.upsert!(:en, 'js.some_key', 'some key (en)')
+          TranslationOverride.upsert!(:fr, 'js.some_key', 'some key (fr)')
+          TranslationOverride.upsert!(:en, 'js.only_en', 'only English')
+          TranslationOverride.upsert!(:fr, 'js.only_fr', 'only French')
+          TranslationOverride.upsert!(:en, 'js.some_client_MF', '{NUM_RESULTS, plural, one {1 result} other {many} }')
+          TranslationOverride.upsert!(:fr, 'js.some_client_MF', '{NUM_RESULTS, plural, one {1 result} other {many} }')
+          TranslationOverride.upsert!(:en, 'js.only_en_MF', '{NUM_RESULTS, plural, one {1 result} other {many} }')
+          TranslationOverride.upsert!(:fr, 'js.only_fr_MF', '{NUM_RESULTS, plural, one {1 result} other {many} }')
+
+          SiteSetting.allow_user_locale = true
+          user = Fabricate(:user, locale: :fr)
+          sign_in(user)
+
+          get "/extra-locales/overrides"
+          expect(response.status).to eq(200)
+
+          ctx = MiniRacer::Context.new
+          ctx.eval("I18n = {};")
+          ctx.eval(response.body)
+
+          overrides = ctx.eval("I18n._overrides")
+          expect(overrides.keys).to contain_exactly("en", "fr")
+          expect(overrides["en"]).to eq({ 'js.only_en' => 'only English' })
+          expect(overrides["fr"]).to eq({ 'js.some_key' => 'some key (fr)', 'js.only_fr' => 'only French' })
+
+          expect(ctx.eval("Object.keys(I18n._mfOverrides)")).to contain_exactly("js.some_client_MF", "js.only_en_MF", "js.only_fr_MF")
         end
       end
     end