Problem
You want to change the default locale.
Solution
Use the Lang::setLocale() method.
Lang::setLocale('es'); // switch to Spanish
Discussion
Difference between App::setLocale() and Lang::setLocale().
Lang::setLocale() only changes the locale in the currently loaded translation service.
App::setLocale() changes the currently loaded configuration value and calls Lang::setLocale() to update the translation service. Also, App::setLocale() fires a locale.changed event.
