In your Lemmy settings (probably need to use the web UI) there should be an option to select your languages. Lemmy will filter out content not marked as your language. Unfortunately, a lot of content is left without a language specified (AKA "undefined") so unless you want to filter out much of the content you do want, you're stuck manually blocking a lot still.

Uns entkommt man nicht

https://lemmy.world/post/8198042

I don't think its possible. I think communities can be set up without a default language.

Same question.