I am currently using Damus, for iOS. Click the magnifying glass, then universal. The feed should only be in the language of your choice.

I do feel like they should give you the option to filter your personal feed as well.
I think the assumption is, if you follow someone, you can understand the language they are using, which is correct. However, that does not account for who they follow and repost. If I have a foreign language message in my feed, it’s usually a repost from someone I am following.