“ma fee Arabi” is a very popular informal way that we’ve heard non-Arabs say, when they want to convey that they don’t speak Arabic.
While this will pretty much get the message across, one of the much more correct ways of saying that you don’t speak Arabic, is to say:
“انا اسف انا لا اتكلم العربية”
Which transliterates to:
“A-na aasifon, a-na laa atha-kallam al-‘arabeeyah”
Which then translates to:
“I am sorry, I don’t speak Arabic.”