"Kio estas la ĉefurbo de Rusio?"
Translation:What is the capital of Russia?
Would kiu be appropriate here? In Bulgarian, I would use коя, which can be translated as kiu or which in English, but it would be odd to use which in English… or not?
"Which" is definitely not right in English. There is often a lot of discussion on whether one should say "kio estas via nomo" or "kiu estas via nomo." This seems like the same question. The short answer is you could probably get away with both.