Use `User#preferred_language` language as default

Instead of using `Gitlab::I18n::AVAILABLE_LANGUAGES` order to define the
user default language in the UI, use the `User#preferred_language`.
`User#preferred_language` is set by default with `I18n.default_locale`.

app/helpers/preferences_helper.rb

@@ -70,7 +70,10 @@ module PreferencesHelper
   end
 
   def language_choices
-    Gitlab::I18n::AVAILABLE_LANGUAGES.map(&:reverse).sort
+    options_for_select(
+      Gitlab::I18n::AVAILABLE_LANGUAGES.map(&:reverse).sort,
+      current_user.preferred_language
+    )
   end
 
   private

changelogs/unreleased/kassio-fix-user-default-language.yml

+---
+title: "Use the user's preferred language as default"
+merge_request: 35676
+author:
+type: fixed

spec/features/profiles/user_edit_preferences_spec.rb

@@ -56,4 +56,17 @@ RSpec.describe 'User edit preferences profile' do
       expect(page).to have_content('Failed to save preferences')
     end
   end
+
+  describe 'User language' do
+    let(:user) { create(:user, preferred_language: :es) }
+
+    it 'shows the user preferred language by default' do
+      expect(page).to have_select(
+        'user[preferred_language]',
+        selected: 'Spanish - español',
+        options: Gitlab::I18n::AVAILABLE_LANGUAGES.values,
+        visible: :all
+      )
+    end
+  end
 end

spec/helpers/preferences_helper_spec.rb

@@ -120,13 +120,6 @@ RSpec.describe PreferencesHelper do
     end
   end
 
-  describe '#language_choices' do
-    it 'returns an array of all available languages' do
-      expect(helper.language_choices).to be_an(Array)
-      expect(helper.language_choices.map(&:first)).to eq(Gitlab::I18n::AVAILABLE_LANGUAGES.values.sort)
-    end
-  end
-
   def stub_user(messages = {})
     if messages.empty?
       allow(helper).to receive(:current_user).and_return(nil)