Change locale detection to run once per session (#8657)

Fix #6462

spec/controllers/concerns/localized_spec.rb

@@ -7,16 +7,10 @@ describe ApplicationController, type: :controller do
     include Localized
 
     def success
-      head 200
+      render plain: I18n.locale, status: 200
     end
   end
 
-  around do |example|
-    current_locale = I18n.locale
-    example.run
-    I18n.locale = current_locale
-  end
-
   before do
     routes.draw { get 'success' => 'anonymous#success' }
   end
@@ -25,19 +19,19 @@ describe ApplicationController, type: :controller do
     it 'sets available and preferred language' do
       request.headers['Accept-Language'] = 'ca-ES, fa'
       get 'success'
-      expect(I18n.locale).to eq :fa
+      expect(response.body).to eq 'fa'
     end
 
     it 'sets available and compatible language if none of available languages are preferred' do
       request.headers['Accept-Language'] = 'fa-IR'
       get 'success'
-      expect(I18n.locale).to eq :fa
+      expect(response.body).to eq 'fa'
     end
 
     it 'sets default locale if none of available languages are compatible' do
       request.headers['Accept-Language'] = ''
       get 'success'
-      expect(I18n.locale).to eq :en
+      expect(response.body).to eq 'en'
     end
   end
 
@@ -48,7 +42,7 @@ describe ApplicationController, type: :controller do
       sign_in(user)
       get 'success'
 
-      expect(I18n.locale).to eq :ca
+      expect(response.body).to eq 'ca'
     end
   end