When saying that I don't know the language a friend taught me:
Я не понимаю по-ру́сски.
On a (German) language learner site they write to use
Извините, я не понял.
when not understanding what was recently said.
I don't comprehend why I should use imperfective when I don't know the language but perfective when I don't understand what was recently said. Thus I continued searching and found:
Я не понимаю, что вы говорите.
Apparently, both is possible using perfective and imperfective. How can I determine in that context when to use the imperfective (понимать) or the perfective (понять)?