I'd say that most dragonewts cannot speak other languages, but some can. Those who need to speak to outsiders can be magically/surgically altered or can use other means, such as pet Newtlings, to speak to outsiders.
As Loz says, it's your game, so if you want some dragonewts to be able to speak the languages of humans then let them.
Hykim and Mhykim are the ancestors of the beasts and are dragons, so it makes sort of sense for some dragonewts to speak Beastspeech, but I wouldn't bother with that. Just give a couple of dragonewts the ability to speak some languages and leave it at that.