Merge branch 'main' into glitch-soc/merge-upstream

Conflicts:
- `README.md`:
  Upstream changed their README, we have our own.
  Kept ours.
- `app/helpers/application_helper.rb`:
  Minor code style fix upstream, on a line that is different in glitch-soc
  due to the different theming system.
  Applied the code style fix to our own code.
- `app/views/settings/preferences/appearance/show.html.haml`:
  Code style fix on a line next to lines exclusive to glitch-soc.
  Applied upstream changes.
- `yarn.lock`:
  Upstream updated a dependency textually close to a glitch-soc-only
  dependency.
  Updated the dependency like upstream did.

app/controllers/api/v1/instances/translation_languages_controller.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class Api::V1::Instances::TranslationLanguagesController < Api::BaseController
+  skip_before_action :require_authenticated_user!, unless: :whitelist_mode?
+
+  before_action :set_languages
+
+  def show
+    expires_in 1.day, public: true
+    render json: @languages
+  end
+
+  private
+
+  def set_languages
+    if TranslationService.configured?
+      @languages = Rails.cache.fetch('translation_service/languages', expires_in: 7.days, race_condition_ttl: 1.hour) { TranslationService.configured.languages }
+      @languages['und'] = @languages.delete(nil) if @languages.key?(nil)
+    else
+      @languages = {}
+    end
+  end
+end