Fix detecting unknown locale

app/Http/Middleware/DetectLanguagePrefer.php

@@ -9,11 +9,21 @@ class DetectLanguagePrefer
 {
     public function handle($request, \Closure $next)
     {
-        $locale = $request->input('lang') ?? $request->cookie('locale') ?? $request->getPreferredLanguage();
+        $locale = $request->input('lang')
-        if (($info = Arr::get(config('locales'), $locale)) && ($alias = Arr::get($info, 'alias'))) {
+            ?? $request->cookie('locale')
+            ?? $request->getPreferredLanguage();
+        if (
+            ($info = Arr::get(config('locales'), $locale)) &&
+            ($alias = Arr::get($info, 'alias'))
+        ) {
             $locale = $alias;
         }
-        app()->setLocale($locale ?? app()->getLocale());
+        $locale ?? app()->getLocale();
+        if (! Arr::has(config('locales'), $locale)) {
+            $locale = config('app.fallback_locale');
+        }
+
+        app()->setLocale($locale);
         Cookie::queue('locale', $locale);
 
         return $next($request);

tests/ServicesTest/TranslationsTest/JavaScriptTest.php

@@ -97,4 +97,11 @@ class JavaScriptTest extends TestCase
             resolve(JavaScript::class)->plugin('en')
         );
     }
+
+    public function testFallbackLocale()
+    {
+        $this->get('/', ['Accept-Language' => 'xyz'])
+            ->assertSuccessful()
+            ->assertSee('lang/en.js');
+    }
 }