Someone correct me if I’m wrong or this has changed, but AFAIK the issue is that Lemmy supports it already but doesn’t require the language tag. Unfortunately if you remove “no language selected” (or whatever it’s called) by it means you don’t see the vast majority of content.

Edit: facepalm I’m just noticing this is a community for the Boost for Lemmy app, not the official site. Please ignore me.

In your account Settings, scroll down to the Languages section and select Undetermined and other languages you’d like to see.

It’s not 100% foolproof, because as others have stated, the language tag isn’t required. But it significantly reduced most of the foreign-language posts I was seeing on the All feed, without impacting the feed in a negative way (AFAIK). For the remainders, you can just block the community if needed.