Use keywords instead of language in source

How to use Crowdin? Tips&Tricks
1

In the source code we use keywords instead of lanugage.
e.g:

cart.title instead of Cart How shall the default language be set up in this case? Since we need a translation file also for the english version. Thank you for your help!
2

Better to have English as source, and let’s say English United States as target language. Having English as both target and source may cause some unexpected confusions in future.

With En + En-US you’ll have 2 different language codes (if you use locale it’ll be en-EN + en-US) so no issues should arise.