Vuejs Laravel always returns validation messages in English despite language switched to 'fr'

 

In my SPA the register form works correctly and displays the validation error messages in English. To get them in an other language, I previously switch the laravel app 's language by mean of a controller that does the following.

public function index(Request $request){
  App::setLocale($request->lang);

  session()->put('locale', $request->lang);
  return response()->json([App::getLocale(),session()->get('locale')]);
}

The returned response is correct

["fr","fr"]

Thus, I am authorized to think that the Laravel app has its language set to French.

In the lang folder, I made a copy of all the stuff under en into a fr folder. Then I translated some of the messages that appear as error when I enter bad values. Unfortunately the messages still appear in English!?

What else should I do? Thanks for your help.