If you say the wrong thing to Arngeir before the quest "Throat of the World", he will refuse to help you. After that Einarth will approach him and speak in dovahzul. "Arngeir. Rek/Rok los Dovahkiin, Strundu'ul. Rek/Rok fen tinvaak Paarthurnax." meaning, "Arngeir. She/He is Dovahkiin, Stormcrown. She/He will speak (with) Paarthurnax".

Also, after the greaybeards proclaim you to be the dovahkiin officially you can ask aregneir what they said, he'll respond with somethin about how most are not as fluent in Dovahzul as they are.

Yeah I saw that and wondered, also I saw that some of the greybeards (borek and wulfgar) will speak to aregneir in dovahzul