For me that'd be Spanish!
I understand a lot of everyday speech, but the grammar wasn't really anything I set myself to learn, so forming sentences both in writing and speech is hard(though I'm guessing the speech part is partly due to my shyness).
So yes, if being unable to speak and write it would be dependent on things such as unwillingness, then I bet there are more than me in that situaton.
It's a bit of the same deal with Japanese as well, since I can understand a bit as I've studied it in school for a bit(and watched a whole lot of anime and other things where you'll stumble upon the language). I mean, I know basic grammar, but writing is supposed to include Kanji(the Chinese symbols) in order to be at least beyond elementary school level. Speaking is also hard, well maybe not, but I'm not really fluent in it.xd