I always set it (mobile client, Thunder), because I find it pretty annoying when I see posts in my feed that I don't understand (so it's only fair that I don't cause it to others)

Fortunately it hasn't been much of an issue on Lemmy, but Mastodon is pretty much unusable for me partly for this reason (last time I tried to curate my feed, ~50% of the posts I saw were in languages I cannot understand -- and I don't follow language-specific topics or people)

It seems it has now been "solved", with a popup for users posting from the website, reminding them to select a language: https://github.com/mastodon/mastodon/issues/25568 I think users (including me) will always make mistakes, and, as you note, not all clients support this setting, so I don't think relying on the UX of everyone's clients is a permanent solution 😕

In the meantime, the best I can do is set the tag manually when I'm posting 😔