According to the Eldar Lexicon entry, the language depends highly on non-verbal communication:
This is largely because, as noted above, the Aeldari language is not actually comprised of words alone like human languages, but also accompanied by a detailed set of poses and gestures. The way an Aeldari stands, the cast of his features or how he moves his hands can all change the meaning of words, sometimes dramatically.
From here.
So, how do Eldar communicate when they're not able to see each other or even with the helmet on? IMHO, they should not be able to just switch this sort of communication of as it's an essential part of their language. Has it been explained how this is done?