What I am thinking is after you learn it you don't forget it. Because even Ron was able to speak some parsel words to open the chamber of secrets passage after hearing them from Harry's sleep talking. So i think it is possible that harry would still remember all the words of that. So that he would be able to speak the language and understand it. Its just like remembering a language with only "S and Sh" s.
PS. It is just like a person doesn't forget English language even after best oblivious charm.