Hi,
This extension is still under development, and I'm still trying to smooth out the rough parts. To make the extension better and to fix the bugs you encountered, I would appreciate some details as to what videos it doesn't filter, like language, and if you noticed a pattern to them. You can leave those details either at the extension's Codeberg (https://codeberg.org/osmo1/youtube-language-filter/issues) or at my email (ylfilter@osmo.zip).
To possibly remedy the issue, you can try installing the YouTube No Translation extension (https://addons.mozilla.org/en-US/firefox/addon/youtube-no-translation/), in case your problem was with auto-translated titles.
Thanks for checking the extension out, and I hope you have a better experience with it in the future!