laravel reponse json message not translated - php

Question

I am using laravel 8 with sanctum api (vue 3 spa).
I have never used localization before....and for this app I need that.
I created a middleware SetLocale with this content
public function handle($request, Closure $next)
{
app()->setLocale(config('app.locale'));
if(session()->has('locale')) {
app()->setLocale(session('locale'));
}
return $next($request);
}
and registered in both web and api (app/Http/Kernel.php) #middlewareGroups
From the login form there is a select that change locale and make a post request to server to change the locale
Session()->put('locale', $request->input('locale'));
app()->setLocale(session('locale'));
return response()->json(['locale' => app()->currentLocale()]);
In the login form the server validation responds with the locale messages.... but the problem is the customized message (Customize laravel sanctum unauthorize response) it is not translated - it gives me in 'en' ... even if the locale is 'de'
$this->renderable(function (\Illuminate\Auth\AuthenticationException $e, $request) {
if ($request->is('api/*')) {
return response()->json([
'message' => trans('_custom.not_authenticated')
], 401);
}
});
The file is there for every language _custom.php (resources/lang/de, resources/lang/en, etc )
example for _custom.php "de":
<?php
return [
'not_authenticated' => 'Nicht berechtigt'
];
The message is still in en ... but if press login (without changing anything) .... the validation errors are in 'de'
Why ? What I missed ?
Could be that the message is returned before the middleware runs ?
*** UPDATE 1 ***
It seems to work partially using in request Accept Language header .... but there is another problem.
In vue I have this approach for the axios:
const api = axios.create({
withCredentials: true,
headers: {
common: {
'Accept-Language': document.documentElement.lang
}
}
});
export default api;
In the vue app when changing the language .... I also update the document.documentElement.lang value ... but when making a request it still the default value 'en'
Is there any way I can change dynamic the value for the 'Accept-Language' header instead of adding for each request the header ?