Are you sure you've selected the option to filter languages, rather than just setting your own interface language? (More info at https://fedi.tips/setting-your-language-preferences-on-mastodon-and-filtering-out-posts-in-other-languages )
The language filter should help at least somewhat, but by definition you won't notice the posts it is successfully hiding.
There is also an automatic language detector, and it tells people when they've got the wrong post language setting, but it's not reliable for short posts so that's why it doesn't override the person's setting.
