Good way to react in a conversation is to say that you are “startled” “astonished” or “surprised”.
You can express these feelings (reactions) with the word 「驚き」 or the phrase 「驚きです」。
That is surprising (to me)
そんなに 日本語を 話せるなんて 驚きです。
It’s a surprise that you can speak Japanese as such (or much as you are “right now”)
= It’s surprising that you can speak that much Japanese.