Kumoko actually understanding bits of the old man's words is different than WN. In WN, IIRC, kumoko just guessed the intention of the old man and her learning the language was something that happened later when she consumed Mother.
I've read that they are differences between WN and LN but I'm already starting to feel like I'm missing something. Especially considering the fact that the anime version is also quite different than WN, can somebody shed some light on this matter?