Steps to reproduce:

1. Go to demoPublic and login as superuser
2. Go to Resources App -> /form/i18n/ and create a new file named form-frontend_de.properties
3. Open the file an include a key like:
   /form/i18n/form-frontend_de.properties

   form.user.errorMessage.email      = GERMAN TEST

4. Logout and open Contact page (switch language to German)
5. Fill the form with an invalid input and check that the error message is in English

Expected result:

The errorMessage shown in a form matches the language of the site

Actual result:

The errorMessage shown in a form matches the anonymous role language

Dev notes:

Switching the language at Security app for anonymous user from English to German shows the proper errorMessage (English or German according the user configuration)