Actually, it depends whether the users take time to tag their toots / posts with the adequate languages. Some users, for example are posting in italian but they forgot to tag their toots with Italian. This is why you'll see Italian, German, French in your timeline even if you applied the language filters.
@BeAware Oh ! That's a nice feature in your app by the way, if your app can guess the language of the toot content instead of relying only on the toot tag language 😅