Should I write app reviews to Mac App Store in English or in my native language?
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.