Threadwielders communicate chiefly corporeally (but not necessarily using sound!) and have a language that is mostly static over the aeons in the sense that words that establish themselves do not suffer semantic shift - but they may and do fall into disuse and are replaced, be extended, or have regional inflections that are narrower in definition than the base word.
Threadwielders immersed in cultures tend to be multi-lingual, but since they cannot expect other Threadwielders to have the same background, their language tends to be maintained as a lingua franca between them, and they fall back to it whenever they visit one another for the first few times.
As such, the language is mostly used by Threadwielders only, though some words will usually sneak into most sapient languages, because of their involvement, especially in mythological contexts.
A notable exception is the kavkema, for whom - due to Evenatra's extreme presence during their cultural genesis - the language was something of a baseline and now continues to be used as a form of 'weakest encryption': While regular conversation is usually in the lingua franca of the Nayabaru (called Naya by the kavkema), conversing in their “ancient tongue” (formally called Kendaneivash) can, in a pinch, buy them a few minutes head-start while someone struggles to translate on-the-fly. (That this “ancient tongue” is still actively expanded by them does not make them change the name. The Nayabaru call it kavkemic, though. Or, you know, “the language those terrorists use”.)