Refactor handling of locales

[marien-probesys]

I've noticed that supported locales are duplicated between app/config/packages/twig.yaml and app/config/services.yaml. Also, we only display the locale code instead of a label in the interface. It could be nice to reorganize things to improve handling of locales.

I've done some experiments in the chore/refactor-locales branch. I'm not sure exactly what still need to be done (probably not much though).

Work started in #339
Vaguely related to #133

Assigning the ticket to @q-desabre as he was interested to finish the work.