My first full time job was at a university, and we had a lot of student interns, most of them not native speakers (lots of chinese students especially). What I've found is that it wasn't worth the trouble to correct every grammar mistake; if a commit message was understandable and had useful content, I'd usually merge it without bothering to correct botched plurals, wrong prepositions, and other mistakes that the students made regularly, as long as it didn't hurt understandability. I found that getting them to write good
content wasn't a problem once we made it clear that this was expected (and it wasn't really easier with the native speakers).
Communication in software development is a huge deal. While language barriers are a thing, I don't think you can or should compromise on documentation, and when it comes to commit messages I don't think this is the leading reason why folks don't write good ones.
I'm told by friends who were at RedHat when they decided to make the source repositories public (as opposed to just throwing release tarballs over the wall), and commit messages went way way up. This tells me: people knew better, they just didn't care if they weren't being held to account.