Right now the app's language is independent from Moodle's language. You can have the app in English but receive the content in French because the user language in the site is French, and that can look weird.
It would be nice to improve that, e.g. by sending the language to the WS so the content is returned in the app's language too (unless it's forced). The main issue here is what happens if the site doesn't have that language installed.