If this were in Japanese, I would be able to convey more meaningful things and impress him/her, but I can only say about 15% of what I want to say.
How frustrating!
ご質問ありがとうございます。
"If this were in Japanese,"＝「これがもし日本語だったら、」
"I would be able to convey more meaningful things and impress him/her,"＝「もっと深い意味があることを伝えて、彼・彼女を感心させることができるのに」
"but I can only say about 15% of what I want to say."＝「言いたいことの15％ぐらいしか言えない。」
"How frustrating!"＝「悔しい！」
ご参考までに。
"日本語だったらもっと深いことが言えるのに"
- If I were speaking Japanese I could say more
- If this conversation were in Japanese, I would be able to contribute more.
「日本語だったらもっと深いこと言ってこの人を感心させられるのに、言いたいことの15%ぐらいしか言えないのが悔しい。」
- It's frustrating that I can only say about 15% of what I want to say, when I could say more in Japanese and impress this person."
アメリカ合衆国 
