Ensure `preferred_language` always have a value

app/models/user.rb

@@ -355,6 +355,12 @@ class User < ApplicationRecord
   scope :order_recent_last_activity, -> { reorder(Gitlab::Database.nulls_last_order('last_activity_on', 'DESC')) }
   scope :order_oldest_last_activity, -> { reorder(Gitlab::Database.nulls_first_order('last_activity_on', 'ASC')) }
 
+  def preferred_language
+    read_attribute('preferred_language') ||
+      I18n.default_locale.to_s.presence_in(Gitlab::I18n::AVAILABLE_LANGUAGES.keys) ||
+      'en'
+  end
+
   def active_for_authentication?
     super && can?(:log_in)
   end

changelogs/unreleased/kassio-ensure-user-preferred-language.yml

+---
+title: Ensure User's preferred_language always has a value.
+merge_request: 37464
+author:
+type: fixed

spec/models/user_spec.rb

@@ -241,6 +241,22 @@ RSpec.describe User do
       it { is_expected.to validate_length_of(:last_name).is_at_most(127) }
     end
 
+    describe 'preferred_language' do
+      context 'when its value is nil in the database' do
+        let(:user) { build(:user, preferred_language: nil) }
+
+        it 'falls back to I18n.default_locale when empty in the database' do
+          expect(user.preferred_language).to eq I18n.default_locale.to_s
+        end
+
+        it 'falls back to english when I18n.default_locale is not an available language' do
+          I18n.default_locale = :kl
+
+          expect(user.preferred_language).to eq 'en'
+        end
+      end
+    end
+
     describe 'username' do
       it 'validates presence' do
         expect(subject).to validate_presence_of(:username)