我看不懂日文,用英语怎么说....

I can't read Japanese
或者
I can't understand Japanese

I don't know Japanese.

I am not good at Japanese.

I can not speak Japanese well.

楼上不对，应该是：
I can't read Japanese
如果是我不会说，应该是：
I can't speak Japanese

I don't know Japanese

I don't speak Japanese. 这才是最地道的(authentic), 通俗易懂

I can't read Japanese