Throw an error in a testing environment when trans('some-key') doesn't exist

strict_localization_lookup.php

<?php

namespace Tests;

use Illuminate\Translation\Translator;
use Laravel\BrowserKitTesting\TestCase as BaseTestCase;

abstract class TestCase extends BaseTestCase
{
    use CreatesApplication;

    protected function setUp()
    {
        parent::setUp();

        $this->strictLocalizationKeys();
    }

    protected function strictLocalizationKeys()
    {
        // if 'some-key' doesn't exist, trans('some-key') will now
        // throw an error, instead of returning 'some-key'.
        $this->app->instance('translator', new class(
            $this->app['translation.loader'],
            $this->app['config']['app.locale']
            ) extends Translator {
                public function get($key, array $replace = [], $locale = null, $fallback = true)
                {
                    $output = parent::get($key, $replace, $locale, $fallback);

                    list($namespace, $group, $item) = $this->parseKey($key);

                    // "validation.php" should be exempt from strictness. Otherwise,
                    // validation breaks when you don't define "custom"
                    // validation messaging.
                    if ($key === $output && ! preg_match('/validation/', $group)) {
                        throw new \Exception(sprintf('Unable to find localization key [%s] in language files.', $key));
                    }

                    return $output;
                }
        });
    }
}