Language fails if default app language is not in list of supported language

[andersespedalen]

When the backup runs, it use its translation strings matching the app locale, but if the default locale of the app is something other than what is supported by the package, you get output i emails like:

backup::notifications.backup_successful_body

Even though the my app have optional english locale, there's no way for the backup to use it.
Setting the locale (App::setLocale('en')) in Console/Kernel.php have no effect.

It would be great to be able to specify in config/backup.php what language to use (whitch is supported by the package), or be able to add your own language files.