Don't worry, Kira, the Doctor had a rather short companion recently, he's used to pacing himself as long as there's nowhere he needs to be. But when she speaks in her native tongue, the Doctor stops. The TARDIS didn't translate that. And it wasn't a language he was familiar with.

He goes down on one knee so he can see her properly.

"That was your language. Can you repeat it? I didn't understand, which is saying something, since I know billions of languages."

In other words, he wants to learn it.