Use browser Accept-Language for login and onboarding locale (#768)

* Use Accept-Language for unauthenticated locale

* Add per-user locale overrides

* Fix test

* Use more than the top `accept-language` entry

* Localization of string

test/controllers/concerns/localize_test.rb

@@ -1,23 +1,55 @@
 require "test_helper"
 
 class LocalizeTest < ActionDispatch::IntegrationTest
-  setup do
-    sign_in users(:family_admin)
+  test "uses Accept-Language top locale on login when supported" do
+    get new_session_url, headers: { "Accept-Language" => "fr-CA,fr;q=0.9" }
+    assert_response :success
+    assert_select "button", text: /Se connecter/i
   end
 
-  test "uses family locale by default" do
-    get preferences_onboarding_url
+  test "falls back to English when Accept-Language is unsupported" do
+    get new_session_url, headers: { "Accept-Language" => "ru-RU,ru;q=0.9" }
+    assert_response :success
+    assert_select "button", text: /Log in/i
+  end
+
+  test "uses Accept-Language for onboarding when user locale is not set" do
+    sign_in users(:family_admin)
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "es-ES,es;q=0.9" }
+    assert_response :success
+    assert_select "h1", text: /Configura tus preferencias/i
+  end
+
+  test "falls back to family locale when Accept-Language is unsupported" do
+    sign_in users(:family_admin)
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "ru-RU,ru;q=0.9" }
     assert_response :success
     assert_select "h1", text: /Configure your preferences/i
   end
 
+  test "respects user locale override even when Accept-Language differs" do
+    user = users(:family_admin)
+    user.update!(locale: "fr")
+    sign_in user
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "es-ES,es;q=0.9" }
+    assert_response :success
+    assert_select "h1", text: /Configurez vos préférences/i
+  end
+
   test "switches locale when locale param is provided" do
+    sign_in users(:family_admin)
+
     get preferences_onboarding_url(locale: "fr")
     assert_response :success
     assert_select "h1", text: /Configurez vos préférences/i
   end
 
   test "ignores invalid locale param and uses family locale" do
+    sign_in users(:family_admin)
+
     get preferences_onboarding_url(locale: "invalid_locale")
     assert_response :success
     assert_select "h1", text: /Configure your preferences/i

test/controllers/users_controller_test.rb

@@ -21,14 +21,15 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
           name: "New Family Name",
           country: "US",
           date_format: "%m/%d/%Y",
-          currency: "USD",
-          locale: "en"
-        }
+          currency: "USD"
+        },
+        locale: "es"
       }
     }
 
     assert_redirected_to settings_profile_url
     assert_equal "Your profile has been updated.", flash[:notice]
+    assert_equal "es", @user.reload.locale
   end
 
   test "admin can reset family data" do