There's no rule about it and I've seen some reviews in Russian and German. But they are always at the end of the reviews section, because people tend to rate them as "not helpful". Could be the best review ever, but it will still get a lot of "not helpful" ratings just because most people don't understand what it says.

I would think it's nice for customers who don't speak English so well to have some reviews in their own (or at least a more similar) language. And, as PaterAlf said, there's no rule against it, so go ahead. Just be aware that the review system is kind of glitchy: it might take a while for a review to show up, and many people have reported that some of their reviews (positive and negative) haven't shown up at all. Nobody's censoring them, it's just GOG's website being its usual terrible self. =)

Those reviews will get down-voted and disappear from the website after a specific number of votes. So, posting them in foreign language it's pretty much useless in the end.

After all, it's normal. Most GOG users can read English. I don't see too much point in having reviews in other languages. Imagine yourself searching over all pages of reviews in order to find a review in your language. Those reviews will be read occasional if they will be ever read by someone that understand the language. It's not common, no one will search for them.

It's hard to have an multi-language website. Pretty much impossible as GOG it's now. Except those 4 languages for menus, news, descriptions, there is no reason to have foreign language content on English setting.

Steam managed to deal with that thing well. You can only see reviews written in the language you have selected. Those in other languages are not visible. That's the only way it can work. If you start mixing them up, it would be annoying to read them or to find the reviews in the language that you want.

As others have said, there's no rule against it, but you're probably wasting your time posting a non-English review. Unless GOG implements a way to filter reviews by language in the future, reviews that aren't in English will be voted down because most users won't be able to understand them.

What he says.