feat: i18n switcher in app-frontend (#4990)

* feat: app i18n stuff

* feat: locale switching on load

* feat: db migration

* feat: polish + fade indicator impl onto TabbedModal

* fix: prepr checks

* fix: remove staging lock for language switching

* fix: lint

apps/frontend/src/locales/en-US/index.json

@@ -2810,30 +2810,6 @@
   "settings.display.theme.title": {
     "message": "Color theme"
   },
-  "settings.language.categories.default": {
-    "message": "Standard languages"
-  },
-  "settings.language.categories.search-result": {
-    "message": "Search results"
-  },
-  "settings.language.description": {
-    "message": "Choose your preferred language for the site. Translations are contributed by volunteers <crowdin-link>on Crowdin</crowdin-link>."
-  },
-  "settings.language.languages.automatic": {
-    "message": "Sync with the system language"
-  },
-  "settings.language.languages.search-field.placeholder": {
-    "message": "Search for a language..."
-  },
-  "settings.language.languages.search-results-announcement": {
-    "message": "{matches, plural, =0 {No languages match} one {# language matches} other {# languages match}} your search."
-  },
-  "settings.language.languages.search.no-results": {
-    "message": "No languages match your search."
-  },
-  "settings.language.warning": {
-    "message": "Changing the site language may cause some content to appear in English if a translation is not available. The site is not yet fully translated, so some content may remain in English for certain languages. We are still working on improving our localization system, so occasionally content may appear broken."
-  },
   "settings.pats.action.create": {
     "message": "Create a PAT"
   },