AH. This depends on how opposed you are to supplementing what we do know with some conjecture, and how willing you are to invent new words within the framework, etc. Salo was quite liberal with his Sindarin invention for Peter Jackson's films, of course. [All of the Khuzdul/Dwarvish is invented except for the inscription on Balin's tomb and the battle cry, 'Barak Khazad! Khazad ai-menû!']My main reason was that you can actually create entire conversations in those languages, which I am not aware of being possible in Quenya or Sindarin since we simply don't have enough vocabulary. I would be very happy to be proven wrong, however!
Quenya is easier and more extensive than Sindarin, but even so, yes, not all ideas can be expressed easily.
However...people have translated the entire New Testament into Quenya, so I think we can manage a bit of dialogue (again, if we're willing to coin words as needed).