And intonation here is very important! You have to kind make the last word`s intonation go down a little. You have to sound like you are in doubt when replying. Another thing you can say where you would not have to worry about intonation as much is:
Ｔｈｅｒｅ ｉｓ ｎｏ ｄｉｒｅｃｔ ｗｏｒｄ ｉｎ Ｅｎｇｌｉｓｈ ｆｏｒ 微妙, so it all depends on the context. It could mean things like tricky, not so good, I dunno, etc. It has a very wide range of meanings, so could you give me a little bit of context, so I could help you more?
"Hmm, I don`t know about that. . ."
So I ask a girl 「今晩、あなたの家に泊まっていい？」
the girl says 「んーそれはちょっと微妙かな。」
So I ask, "Hey girl, can I stay at you house tonight?" Then she says,
"Hmmm, I dunno (I don`t know). . ." Which means "no" of course.
「俺の作った料理、おいしい？」
「んーそれはちょっと微妙だな」
Ｉｓ ｔｈｅ ｆｏｏｄ Ｉ ｍａｄｅ ｇｏｏｄ？
Ｕｈ， Ｉ ｄｕｎｎｏ. . .
OR
eh. . . sort of
ＯＲ
Well. . . It`s not great. . .
So if you use 微妙 like this, most of the time "I don`t know" will work.
I hope this helped a little bit!
でも、英語ではそれにぴったり相応する言葉はないみたいです。
味が微妙、だとか、判断が微妙だとか、雰囲気が微妙だとか、"何が微妙か″状況により、表現が変るみたいです。