In vendor/laravel/lumen-framework/src/helpers.php:363 a trans($id = null, $parameters = [], $domain = 'messages', $locale = null) function is defined, so you can use it in your views
If you don't request for a certain lang translation, it uses dotenv's APP_LOCALE setting defined in you .env file to get the localised values.
Using Blade:
trans('file.key')
(sorry, I failed to get blade syntax to be highlighted as code, just wrap it in two brackets as usual)