How do I change the original language in PHP files and Twig files?
How do I tell Drupal that all my strings (PHP and Twig) are French strings?


You can not. There is no setting for that, the source code must be english.

Drupal does allow to translate the code/system-english to the actual english if you enable the checkbox when editing the english language. I don't know what that does exactly to potx and I don't think TMGMT supports it either.