As far as I can tell, it just uses whatever language you use Steam in. If that language doesn’t have enough reviews, it defaults to all languages.

I also just tried and set German as a secondary language, but it still just shows the score for English reviews. It doesn’t even mention German at the bottom of the page, where you get the summary, only all languages and English (and recent). If I change the language for the store, it also changes the source for the review score.