Hi @pedromvieira!
Thank you for your answer. I finally found what I was looking for! Here are the solutions, should anyone looking to achieve similar results need them:
Based on this issue: Running mix gettext.extract in umbrella updates wrong app · Issue #126 · elixir-gettext/gettext · GitHub, I was able to find this working exemple umbrella app from wojtekmach: https://github.com/wojtekmach/gettext_umbrella_bug.
I then ran into this issue: Translations are dissapearing in umbrella · Issue #178 · elixir-gettext/gettext · GitHub, which I was able to solve thank’s to amatalai’s idea: Workaround for gettext.extract issue with umbrella app · GitHub
I now have an umbrella app containing a Translator app, which is responsible for handling the translations for all the other apps (some of them being phoenix apps). The only app depending on Gettext is the Translator app, which is an :in_umbrella dependency to any app in need of translations. Using a Mix task inspired from amatalai’s, every app extracts its translations into the Translator app.
May these solutions save some headaches in the future! 
Cheers! 
