Dialects =/= languages. Here's what "Drow language" is, right out of "Drow of the Underdark", p. 38:Drow is a language. It's a mix of elven, undercommon and other words picked up here and there. Of course Rapsam had to say I was wrong and give some kind of counter argument, even though learning drow or understanding little bits of drow here and there cuz somebody knows elven isn't the same thing.
Huh, weird. Drow don't have their own language...
If the actual sourcebook doesn't put to bed this whole thread, then frankly we may as well as homebrew everything and start acting like Haven with our own custom lore. /sarcasm
On a serious note, if we want to rule one must know Undercommon and Elvish to know "Drow speech", fine. But that's a staff call.