It does work but not 100% which makes it feel like it doesn't. Language detection is very far from totally accurate (we use the CLD3 library), so sometimes it gives the wrong language. People can also just select which language they post in because when that's done in good faith it's more reliable than language detection, but of course it can also lead to some posts not being in the language they're supposed to be in. Using language filters cuts down on a lot of posts though, even if some slip through.
I'm on few international instances and always had German and French pop up even-though I have only english checked; Could it be that the instances themselves are faulty here?
I guess it makes sense that the filter would be confused on tech based talk as it includes a lot of english loan-words though it's a bit irritating when browsing public timelines.
Language detection is more accurate on longer texts, short messages have a lot of false positives and many English loan-words would only worsen the situation indeed.