Wouldn't it be much easier if there was a core supported concept to define a fallback?
I locally achieved this by introducing a "fallback" key in the extra section of the translations composer.json to register one or multiple fallbacks (default: en).
Using this approach you just need to add the default language variant as a required package and you are golden.
Symfony already supports language fallbacks btw
PS: Would love to post a PR to GitHub but I am currently unable to do so until my ISP stops screwing me.