change language

Check https://codex.www.ads-software.com/WordPress_in_Your_Language

Then browse to a link in you language and follow the instructions.

Thanks, but I already have translated everything myself. I just need to change the months for the posts, calendar and archive into another language. Do you know in what files I can find them, please ?

Found the answer: it can be changed in this file: wp-includes/locale.php

Keep in mind that after WP upgrade you’ll have to do it again. It’s better to play with PO files than with core WP.