The Understanding
The Understanding is an idea that tries to explain the fact that sapient inhabitants of Noih can understand each other's speech. Adherents to it claim that it is a gift given to them by the gods.
It is unknown how it's actually possible that people can talk with each other no matter the species. Limited research has been conducted into how language information transfers, and the most accepted theory seems to be that there is a universal web of conscious thought that connects all people, and by that abstract thoughts can be translated across wildly different minds and brains. This applies to written things as well.
However, the research so far has been done at a small scale, and it has mostly focused on written language, leaving spoken language a continued mystery. It's unknown how exactly people learn to talk, for instance.