Add user's preferred language to cache key of MR title partial

This is to fix a bug wherein the Edit button is being shown in a
different language from the current user's preferred language.

Before this fix, when the partial was cached and the language is
different, it'll stay that way until the cache is invalidated.

Now, the current user's preferred language will be taken into
consideration for the cache key.

Changelog: fixed

app/views/projects/merge_requests/_mr_title.html.haml

@@ -2,7 +2,7 @@
 - can_update_merge_request = can?(current_user, :update_merge_request, @merge_request)
 - can_reopen_merge_request = can?(current_user, :reopen_merge_request, @merge_request)
 - are_close_and_open_buttons_hidden = merge_request_button_hidden?(@merge_request, true) && merge_request_button_hidden?(@merge_request, false)
-- cache_key = [@project, @merge_request, can_update_merge_request, can_reopen_merge_request, are_close_and_open_buttons_hidden]
+- cache_key = [@project, @merge_request, can_update_merge_request, can_reopen_merge_request, are_close_and_open_buttons_hidden, current_user&.preferred_language]
 
 = cache(cache_key, expires_in: 1.day) do
   - if @merge_request.closed_or_merged_without_fork?

spec/features/merge_request/user_views_open_merge_request_spec.rb

@@ -128,4 +128,30 @@ RSpec.describe 'User views an open merge request' do
       expect(find("[data-testid='ref-name']")[:title]).to eq(source_branch)
     end
   end
+
+  context 'when user preferred language has changed', :use_clean_rails_memory_store_fragment_caching do
+    let(:project) { create(:project, :public, :repository) }
+    let(:user) { create(:user) }
+
+    before do
+      project.add_maintainer(user)
+      sign_in(user)
+    end
+
+    it 'renders edit button in preferred language' do
+      visit(merge_request_path(merge_request))
+
+      page.within('.detail-page-header-actions') do
+        expect(page).to have_link('Edit')
+      end
+
+      user.update!(preferred_language: 'de')
+
+      visit(merge_request_path(merge_request))
+
+      page.within('.detail-page-header-actions') do
+        expect(page).to have_link('Bearbeiten')
+      end
+    end
+  end
 end