Should I write app reviews to Mac App Store in English or in my native language? While browsing the apps in Mac App Store I can see the majority of the reviews are posted by users with Finnish sounding names. Some of the reviews are written in English and some in Finnish.

I have set the App Store's language as Finnish. Does App Store show only reviews from users that have set the same language? Should I write my reviews in English or in Finnish, ie. in my native language?

Confusingly enough, the most of the GUI elements in App Store are in English, with some Finnish exceptions — this may be the reason there are so many reviews written in English by Finns.
App Store is divided by countries it is available in and each country's App Store is a separate entity. For an end-user (me, you, anyone who buys apps basically) there is no cross-over between them. If you're from Finland you're most likely using Finnish App Store (possibly with interface in English).

Only users of the same store will see the reviews posted in it. Reviews are not shared between individual (country) stores so it is perfectly fine for reviews to be in country's native language.