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!
This is the first time I've heard you speak Japanese.
日本語の「〜を初めて聞く」を直訳するよりも、This is the first time + 現在完了形 の構文を使うのが最も自然です。
This is the first time I've heard you speak Japanese. 『（今話しているのを聞いて）あなたが日本語を話すのを初めて聞きました。』
This is the first time...
「これは初めての時だ」という構造で、現在の経験（今聞いていること）が初めてであることを強調します。
アメリカ合衆国 
