Your friend talks to somebody in Japanese. You're surprised, "I didn't know you can speak Japanese!" Then say, "I hear you speak Japanese FOR THE FIRST TIME"
Is there any way to say more naturally?
This is the first time I've heard you speak Japanese.
Thanks for your question.
The most natural way to say this is, "This is the first time I've heard you speaking Japanese."
Another way you can say this is, "I'm hearing you speak Japanese for the first time."
I hope this helps!
アメリカ合衆国 
