Descriptions of YouTube links posted to lemmy.ml are in French. This seems to depend on the instance rather than the user viewing the post.
I'm guessing, the instance server fetches this description once and YouTube responds with a language based on the server IP address.
...and I just checked, yep, according to one of the IP geolocation services, lemmy.ml is hosted in France, midwest.social is hosted in Germany.
I recall that some Mastodon instances were all showing YT links in German. It was a YT problem at one point because even Threads was having similar issues. Not sure if that was 100% fixed yet.