diff --git a/lib/i18n/backend/fallback_locale_list.rb b/lib/i18n/backend/fallback_locale_list.rb
index e1b3d25ebbb..f1b5acaea6e 100644
--- a/lib/i18n/backend/fallback_locale_list.rb
+++ b/lib/i18n/backend/fallback_locale_list.rb
@@ -6,19 +6,15 @@ module I18n
     class FallbackLocaleList < Hash
       def [](locale)
         locale = locale.to_sym
-        return [locale] if locale == :en
+        locale_list = [locale]
+        return locale_list if locale == :en
 
-        fallback_locale = LocaleSiteSetting.fallback_locale(locale)
-        site_locale = SiteSetting.default_locale.to_sym
-
-        locale_list =
-          if locale == site_locale || site_locale == :en || fallback_locale == :en
-            [locale, fallback_locale, :en]
-          else
-            site_fallback_locale = LocaleSiteSetting.fallback_locale(site_locale)
-            [locale, fallback_locale, site_locale, site_fallback_locale, :en]
-          end
+        while (fallback_locale = LocaleSiteSetting.fallback_locale(locale))
+          locale_list << fallback_locale
+          locale = fallback_locale
+        end
 
+        locale_list << :en
         locale_list.uniq.compact
       end
     end
diff --git a/spec/components/js_locale_helper_spec.rb b/spec/components/js_locale_helper_spec.rb
index c1b9933f080..e1c9cb40bba 100644
--- a/spec/components/js_locale_helper_spec.rb
+++ b/spec/components/js_locale_helper_spec.rb
@@ -130,7 +130,7 @@ describe JsLocaleHelper do
     end
   end
 
-  it 'performs fallbacks to english if a translation is not available' do
+  it 'performs fallbacks to English if a translation is not available' do
     JsLocaleHelper.set_translations('en', "en" => {
         "js" => {
           "only_english" => "1-en",
@@ -161,12 +161,12 @@ describe JsLocaleHelper do
     expected = {
       "none" => "[uk.js.none]",
       "only_english" => "1-en",
-      "only_site" => "2-ru",
-      "english_and_site" => "3-ru",
+      "only_site" => "[uk.js.only_site]",
+      "english_and_site" => "3-en",
       "only_user" => "4-uk",
       "english_and_user" => "5-uk",
       "site_and_user" => "6-uk",
-      "all_three" => "7-uk",
+      "all_three" => "7-uk"
     }
 
     SiteSetting.default_locale = 'ru'
@@ -178,9 +178,9 @@ describe JsLocaleHelper do
     ctx.eval(JsLocaleHelper.output_locale(I18n.locale))
     ctx.eval('I18n.defaultLocale = "ru";')
 
-    expect(ctx.eval('I18n.translations.en.js').keys).to contain_exactly("only_english")
-    expect(ctx.eval('I18n.translations.ru.js').keys).to contain_exactly("only_site", "english_and_site")
+    expect(ctx.eval('I18n.translations').keys).to contain_exactly("uk", "en")
     expect(ctx.eval('I18n.translations.uk.js').keys).to contain_exactly("all_three", "english_and_user", "only_user", "site_and_user")
+    expect(ctx.eval('I18n.translations.en.js').keys).to contain_exactly("only_english", "english_and_site")
 
     expected.each do |key, expect|
       expect(ctx.eval("I18n.t(#{"js.#{key}".inspect})")).to eq(expect)
diff --git a/spec/lib/i18n/discourse_i18n_spec.rb b/spec/lib/i18n/discourse_i18n_spec.rb
index 0215dc33fee..39c0691d363 100644
--- a/spec/lib/i18n/discourse_i18n_spec.rb
+++ b/spec/lib/i18n/discourse_i18n_spec.rb
@@ -13,7 +13,6 @@ describe I18n::Backend::DiscourseI18n do
     backend.store_translations(:en, foo: 'Foo in :en', bar: 'Bar in :en', wat: 'Hello %{count}')
     backend.store_translations(:en, items: { one: 'one item', other: '%{count} items' })
     backend.store_translations(:de, bar: 'Bar in :de')
-    backend.store_translations(:ru, baz: 'Baz in :ru')
     backend.store_translations(:en, link: '[text](url)')
   end
 
@@ -54,13 +53,6 @@ describe I18n::Backend::DiscourseI18n do
       expect(backend.translate(:de, 'bar')).to eq('Bar in :de')
       expect(backend.translate(:de, 'foo')).to eq('Foo in :en')
     end
-
-    it 'uses default_locale as fallback when key exists' do
-      SiteSetting.default_locale = 'ru'
-      expect(backend.translate(:de, 'bar')).to eq('Bar in :de')
-      expect(backend.translate(:de, 'baz')).to eq('Baz in :ru')
-      expect(backend.translate(:de, 'foo')).to eq('Foo in :en')
-    end
   end
 
   describe '#exists?' do
@@ -75,7 +67,6 @@ describe I18n::Backend::DiscourseI18n do
     it 'returns true when an existing key and an existing locale is given' do
       expect(backend.exists?(:en, :foo)).to eq(true)
       expect(backend.exists?(:de, :bar)).to eq(true)
-      expect(backend.exists?(:ru, :baz)).to eq(true)
     end
 
     it 'returns false when a non-existing key and an existing locale is given' do
diff --git a/spec/lib/i18n/fallback_locale_list_spec.rb b/spec/lib/i18n/fallback_locale_list_spec.rb
index 793ac8b2a0a..f84a4908aa7 100644
--- a/spec/lib/i18n/fallback_locale_list_spec.rb
+++ b/spec/lib/i18n/fallback_locale_list_spec.rb
@@ -16,7 +16,7 @@ describe I18n::Backend::FallbackLocaleList do
   it "works when default_locale is English (United States)" do
     SiteSetting.default_locale = :en_US
 
-    expect(list[:ru]).to eq([:ru, :en_US, :en])
+    expect(list[:ru]).to eq([:ru, :en])
     expect(list[:en_US]).to eq([:en_US, :en])
     expect(list[:en]).to eq([:en])
   end
@@ -24,7 +24,7 @@ describe I18n::Backend::FallbackLocaleList do
   it "works when default_locale is not English" do
     SiteSetting.default_locale = :de
 
-    expect(list[:ru]).to eq([:ru, :de, :en])
+    expect(list[:ru]).to eq([:ru, :en])
     expect(list[:de]).to eq([:de, :en])
     expect(list[:en]).to eq([:en])
     expect(list[:en_US]).to eq([:en_US, :en])
@@ -34,6 +34,7 @@ describe I18n::Backend::FallbackLocaleList do
     before do
       DiscoursePluginRegistry.register_locale("es_MX", fallbackLocale: "es")
       DiscoursePluginRegistry.register_locale("de_AT", fallbackLocale: "de")
+      DiscoursePluginRegistry.register_locale("de_AT-formal", fallbackLocale: "de_AT")
     end
 
     after do
@@ -44,15 +45,27 @@ describe I18n::Backend::FallbackLocaleList do
       SiteSetting.default_locale = :en
 
       expect(list[:de_AT]).to eq([:de_AT, :de, :en])
+      expect(list[:"de_AT-formal"]).to eq([:"de_AT-formal", :de_AT, :de, :en])
       expect(list[:de]).to eq([:de, :en])
       expect(list[:en]).to eq([:en])
     end
 
+    it "works when default_locale is English (United States)" do
+      SiteSetting.default_locale = :en_US
+
+      expect(list[:de_AT]).to eq([:de_AT, :de, :en])
+      expect(list[:"de_AT-formal"]).to eq([:"de_AT-formal", :de_AT, :de, :en])
+      expect(list[:de]).to eq([:de, :en])
+      expect(list[:en]).to eq([:en])
+      expect(list[:en_US]).to eq([:en_US, :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[:es_MX]).to eq([:es_MX, :es, :en])
+      expect(list[:"de_AT-formal"]).to eq([:"de_AT-formal", :de_AT, :de, :en])
+      expect(list[:es]).to eq([:es, :en])
       expect(list[:en]).to eq([:en])
     end
   end
diff --git a/spec/models/theme_field_spec.rb b/spec/models/theme_field_spec.rb
index 570a861d7a9..fcc91536f4b 100644
--- a/spec/models/theme_field_spec.rb
+++ b/spec/models/theme_field_spec.rb
@@ -8,6 +8,10 @@ describe ThemeField do
     ThemeField.destroy_all
   end
 
+  before do
+    I18n.locale = :en
+  end
+
   describe "scope: find_by_theme_ids" do
     it "returns result in the specified order" do
       theme = Fabricate(:theme)
@@ -60,22 +64,22 @@ describe ThemeField do
 
   it 'only extracts inline javascript to an external file' do
     html = <<~HTML
-    <script type="text/discourse-plugin" version="0.8">
-      var a = "inline discourse plugin";
-    </script>
-    <script type="text/template" data-template="custom-template">
-      <div>custom script type</div>
-    </script>
-    <script>
-      var b = "inline raw script";
-    </script>
-    <script type="texT/jAvasCripT">
-      var c = "text/javascript";
-    </script>
-    <script type="application/javascript">
-      var d = "application/javascript";
-    </script>
-    <script src="/external-script.js"></script>
+      <script type="text/discourse-plugin" version="0.8">
+        var a = "inline discourse plugin";
+      </script>
+      <script type="text/template" data-template="custom-template">
+        <div>custom script type</div>
+      </script>
+      <script>
+        var b = "inline raw script";
+      </script>
+      <script type="texT/jAvasCripT">
+        var c = "text/javascript";
+      </script>
+      <script type="application/javascript">
+        var d = "application/javascript";
+      </script>
+      <script src="/external-script.js"></script>
     HTML
 
     theme_field = ThemeField.create!(theme_id: 1, target_id: 0, name: "header", value: html)
@@ -91,13 +95,13 @@ describe ThemeField do
 
   it 'adds newlines between the extracted javascripts' do
     html = <<~HTML
-    <script>var a = 10</script>
-    <script>var b = 10</script>
+      <script>var a = 10</script>
+      <script>var b = 10</script>
     HTML
 
     extracted = <<~JavaScript
-    var a = 10
-    var b = 10
+      var a = 10
+      var b = 10
     JavaScript
 
     theme_field = ThemeField.create!(theme_id: 1, target_id: 0, name: "header", value: html)
@@ -301,8 +305,8 @@ HTML
     let!(:theme3) { Fabricate(:theme) }
 
     let!(:en1) {
-      ThemeField.create!(theme: theme, target_id: Theme.targets[:translations], name: "en_US",
-                         value: { en_US: { somestring1: "helloworld", group: { key1: "enval1" } } }
+      ThemeField.create!(theme: theme, target_id: Theme.targets[:translations], name: "en",
+                         value: { en: { somestring1: "helloworld", group: { key1: "enval1" } } }
                                   .deep_stringify_keys.to_yaml
       )
     }
@@ -313,21 +317,21 @@ HTML
       )
     }
     let!(:fr2) { ThemeField.create!(theme: theme2, target_id: Theme.targets[:translations], name: "fr", value: "") }
-    let!(:en2) { ThemeField.create!(theme: theme2, target_id: Theme.targets[:translations], name: "en_US", value: "") }
+    let!(:en2) { ThemeField.create!(theme: theme2, target_id: Theme.targets[:translations], name: "en", value: "") }
     let!(:ca3) { ThemeField.create!(theme: theme3, target_id: Theme.targets[:translations], name: "ca", value: "") }
-    let!(:en3) { ThemeField.create!(theme: theme3, target_id: Theme.targets[:translations], name: "en_US", value: "") }
+    let!(:en3) { ThemeField.create!(theme: theme3, target_id: Theme.targets[:translations], name: "en", value: "") }
 
     describe "scopes" do
       it "filter_locale_fields returns results in the correct order" do
         expect(ThemeField.find_by_theme_ids([theme3.id, theme.id, theme2.id])
           .filter_locale_fields(
-           ["en_US", "fr"]
+           ["en", "fr"]
         )).to eq([en3, en1, fr1, en2, fr2])
       end
 
       it "find_first_locale_fields returns only the first locale for each theme" do
         expect(ThemeField.find_first_locale_fields(
-          [theme3.id, theme.id, theme2.id], ["ca", "en_US", "fr"]
+          [theme3.id, theme.id, theme2.id], ["ca", "en", "fr"]
         )).to eq([ca3, en1, en2])
       end
     end
@@ -358,7 +362,7 @@ HTML
       it "loads correctly" do
         expect(fr1.translation_data).to eq(
           fr: { somestring1: "bonjourworld", group: { key2: "frval2" } },
-          en_US: { somestring1: "helloworld", group: { key1: "enval1" } }
+          en: { somestring1: "helloworld", group: { key1: "enval1" } }
         )
       end
 
@@ -380,7 +384,7 @@ HTML
         theme.reload
         expect(fr1.translation_data).to eq(
           fr: { somestring1: "bonjourworld", group: { key2: "frval2" } },
-          en_US: { somestring1: "helloworld", group: { key1: "overriddentest1" } }
+          en: { somestring1: "helloworld", group: { key1: "overriddentest1" } }
         )
       end
     end
@@ -398,9 +402,9 @@ HTML
     describe "prefix injection" do
       it "injects into JS" do
         html = <<~HTML
-        <script type="text/discourse-plugin" version="0.8">
-          var a = "inline discourse plugin";
-        </script>
+          <script type="text/discourse-plugin" version="0.8">
+            var a = "inline discourse plugin";
+          </script>
         HTML
 
         theme_field = ThemeField.create!(theme_id: theme.id, target_id: 0, name: "head_tag", value: html)
diff --git a/spec/models/theme_spec.rb b/spec/models/theme_spec.rb
index ae162773e37..04b452ccfc6 100644
--- a/spec/models/theme_spec.rb
+++ b/spec/models/theme_spec.rb
@@ -7,6 +7,10 @@ describe Theme do
     Theme.clear_cache!
   end
 
+  before do
+    I18n.locale = :en
+  end
+
   fab! :user do
     Fabricate(:user)
   end
@@ -657,8 +661,8 @@ HTML
     end
 
     it "can create a hash of overridden values" do
-      en_translation = ThemeField.create!(theme_id: theme.id, name: "en_US", type_id: ThemeField.types[:yaml], target_id: Theme.targets[:translations], value: <<~YAML)
-        en_US:
+      en_translation = ThemeField.create!(theme_id: theme.id, name: "en", type_id: ThemeField.types[:yaml], target_id: Theme.targets[:translations], value: <<~YAML)
+        en:
           group_of_translations:
             translation1: en test1
       YAML
@@ -668,7 +672,7 @@ HTML
       theme.update_translation("group_of_translations.translation1", "overriddentest2")
       theme.reload
       expect(theme.translation_override_hash).to eq(
-        "en_US" => {
+        "en" => {
           "group_of_translations" => {
             "translation1" => "overriddentest1"
           }