Hi Johan welcome to the forum
Setting a site’s Locale will change only the text that has tokens and values in the yml file.
It will not translate everything, eg. existing posts made in another language, Setting values.
eg. notice that “Utilisez une classe CSS Font Awesome ou une URL d’image” and “Groupe” display in French.
The “long description” is not part of the Discourse UI and therefore is not in the yml file. It is in a Postgres table.
IMHO likely easiest to simply change it to French yourself if that’s what you prefer it to be.
I wouldn’t trust myself at attempting to do any type of translating via a query.
That it doesn’t display “longue” suggests that the Transifex file needs to be updated.