Gallery: allow to change the default language

[g123k]

Hi everyone!

I've noticed that we have more and more products with English as the default language (which is wrong).
We can change this default language on the website, and it would be nice to also be able to change it in the app.

[teolemon]

• Related to Implement the API to change the default language of a product openfoodfacts-dart#1015 where I answered

[monsieurtanuki]

Impacts:

1. No more systematically setting the "main" language in BackgroundTaskDetails
   => may impact setting the translated texts - that's why I added that line of code years ago
2. Should we display the "main" language in the product page?
   => for the record it's not the case in the website
3. In which part of the app should we make the "main" language editable