Change language for Laravel and Vue incl. validation

I used inertiajs for the combination of Laravel and Vue. But the basic procedures remain the same.

Setup i18n-vue
The configuration and usage assumes the use of the Option API of Vue. For the composition API, the setup looks different. And this.$i18n.locale cannot be accessed directly. See then https://vue-i18n.intlify.dev/guide/advanced/composition.html#basic-usage
app.js

createInertiaApp({
    title: (title) => `${title} - ${appName}`,
    resolve: (name) => require(`./Pages/${name}.vue`),
    setup({ el, app, props, plugin }) {
        const i18n = createI18n({
            locale: "de",
            // locale: props.initialPage.props.locale, // user locale by props
            fallbackLocale: "en", // set fallback locale
            messages: loadLocaleMessages(),
        });

        return createApp({ render: () => h(app, props) })
            .use(plugin)
            .use(i18n)
            .mixin({ methods: { route } })
            .mount(el);
    },
});

The Laravel locale is set via a LanguageMiddleware. From the session or if it is missing from the config.

class LanguageMiddleware
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure(\Illuminate\Http\Request): (\Illuminate\Http\Response|\Illuminate\Http\RedirectResponse)  $next
     * @return \Illuminate\Http\Response|\Illuminate\Http\RedirectResponse
     */
    public function handle(Request $request, Closure $next)
    {
        if(session()->has('locale')){
            app()->setLocale(session('locale'));
        } else {
            app()->setLocale(config('app.locale'));
        }
        return $next($request);
    }
}

The LanguageMiddleware must be declared in the kernel:

...
protected $middlewareGroups = [
        'web' => [
            ...
            \App\Http\Middleware\LanguageMiddleware::class,
        ],

    ];
...

In the Web Routes the route is defined to change the locale in the session.
web.php

Route::put('/laravel-language/{key}', function ($key) {
    session()->put('locale', $key);
    return redirect()->back();
});

The locale switcher is displayed in the frontend:
Authenticated.vue

  <div v-if="$i18n.locale == 'en'">
      <a @click.prevent="switchLanguage('de')" href="#"> Deutsch </a>
  </div>
  <div v-if="$i18n.locale == 'de'">
      <a @click.prevent="switchLanguage('en')" href="#"> English </a>
  </div>

and the method switchLanguage is called. The method changes both the locale of i18n-vue and calls the route to change the Laravel locale.

<script>
export default {
    methods: {
        switchLanguage(key) {
            this.$i18n.locale = key;
            this.$inertia.put('/laravel-language/' + key);
        },
    },
}
</script>