Okay, but like-
Tianyi's concept is that she's an alien who can't speak "human language" (which is, in her case, Chinese), and so she can only make her feelings and thoughts known through the universal language of music. (Yeah, I know it's a mostly-abandoned concept, but it's still fun.)
Xingchen, having been birthed from the cosmos (/ZERO), has been stated to be able to travel between stars and worlds. When on Earth, she and her sisters speak Chinese, but there's no reason she couldn't know other languages from other worlds.
Tianyi meeting Xingchen. Xingchen tries striking up a conversation with Tianyi, so Tianyi tries telling her in her own language that she can't speak Chinese. So Xingchen, without missing a beat, says in perfect Tianyish, "Okay, we'll speak in your language!" and Tianyi's just. Shook. Because that's the first time she's ever heard her language come out of someone else's mouth since she left her home world.
They'll have extensive conversations with each other in their alien language and the Vsingers are simultaneously amazed and annoyed because Moke's pretty sure he just heard Tianyi say his name, and now both girls are laughing...