In your settings, you can set your preferred languages - since you’re on lemmy.world this link should take you there. You can hold control down to select multiple options if you’re on desktop (and mobile should have a selection box pop up).

Though be sure to keep “undetermined” enabled, otherwise a lot of stuff will vanish. Also note that this only works for posts/comments where people have actually selected the correct language (if someone posts something in German, but marks it as English for example, it will still pop up).

Yes. In your desktop settings, hold ctrl and click on every language you want to see. It’s best to keep undetermined because some people don’t tag what language their content is in. What you’re probably seeing are these untagged posts, because I think registering your account asks which languages you want to see in the first place.

Best way at the moment is to filter out foreign language communities, as most of these posts are not in the general communities.