I know I’m late to the party but I just found out and didn’t find a post regarding this
TL;DR: Steam now calculates review scores based on reviews made on your languages only (if enough). This is on by default, but can be reverted to go back to review scores based on all reviews (from people that purchased the game directly on Steam only)
So they’re not actually checking the review for the language used. So they must be storing a tag with the review as to what language that reviewers steam is set to. I can see problems with this right there, a lot of people will have steam in whatever their desired language is, say German, but will post reviews in English. It seems like these reviews won’t be taken into account because they’ll be incorrectly tagged as German.