While almost every TBK knows elvish, Adaine and Fabian use it more.
Adaine “correctly” pronounces his name when she’s trying to do something nice for him; a pep talk, giving gifts (see their gift exchange)
Fabian “correctly” pronounces her name when he REALLY needs her to hear him; when he’s in danger, when he’s trying to apologize about something
Fabian knows when he needs to apologize to her when she starts speaking elvish continuously. So he waits for a moment then he busts out “A-dayne”. When he hears “Fah-byan”, he knows she’s in the forgiving mode already.