Doing some thread necromancy here, but from what I can gather it sounds like Monolingual respects app signing and won't remove files that would invalidate the app's signature, which is actually part of the reason why it's less effective than it used to be. That, and its inability to remove system files due to them being read only.
Anyway, it's 2023 and Monolingual still works so far as languages are concerned (though it only saves a fraction of the space it used to), but I'd be hesitant to use the architecture removal functionality on an M-series chip given that it could break Intel-only apps that run under Rosetta. The app also hasn't yet been updated to be Universal, so it, itself is also running under Rosetta. It's been one of the first apps I've installed on every Mac I've run for the last decade or two, but at this point I'm thinking I may finally retire it, what with its benefits getting smaller while storage gets cheaper.
EDIT: Just noticed that the dev has links in various GitHub issues to a pre-release 1.9 version, so maybe it isn't done for just yet?