Francais – Polylang allows you to translate the text attached to your media, for example the title, the alternative text, the caption, or the description. When you translate a media, the file is not duplicated on your disk, however you will see one entry per language in the media library. When you want to insert media in a post, only the media in the language of the current post will be displayed.
You must activate media translation if you want to translate the title, the alternative text, the caption, or the description. Otherwise you can safely deactivate it.