Polylang Case 1: You activated the languages and translations for the media.
When editing a post, only the media in the same language as the post you are editing are visible in the media library. You must translate your media in the post’s language.
Case 2: You deactivated the languages and translations for the media.
If you previously uploaded media while the language and translation for media was active, they will keep their language and won’t be visible in other languages.
Thus, as explained in the media documentation, it is not recommended to change the media setting once you started to work with media.