There isn't a proper translation. The translation changes depending on the context in which it is used, keep in mind that it is a negation, also used to form rhetorical questions or a reinforcement (I prefer to call it "modulator") of denial. To translate that, in terms of meaning, you can use many adverbs or sentence structures. As far as I know, there are no words in English that also have the informal connotation of "mica" and its "rhetorical value".
Let's start with easy examples of how to modulate the negation:
Non glielo dico mica! | (Surely) I'm not going to tell him about that!
Non capisco mica la tua domanda. | I don't understand at all your question.
Mica male! | Not bad! (In this case maybe you could use "Quite good!", or I found this translation "It’s not hardly that bad." too.)
Mica male questa domanda. | Not bad this question.
"mica" can be used in the context of fear that something that we don't want is happening, hoping for a negative answer fearing for an affirmative one. Read these examples (the translation is quite hard so don't take it as good ones):
Non mi denuncerà mica per questo? | He won't sue me for that, will he?
Non si sarà mica fatto male! | He wouldn't hurt himself, has he?
Non sarà mica partito senza il casco! | I hope that he has not left without his helmet!
Now the next are more difficult to understand because there is the rethorical meaning, this first one suppose that we take into account the possibility of a negative answer:
Hai mica una sigaretta? | by any chance would you have a cigarette?
The sense you should try to convey is that the person who is using "mica" knows that the person is talking with thought a determinate things that it isn't true.
Non voglio mica fregarti! | (I know that you think so) but I really don't want to rip you off!
For me even this one have this sense:
Non gli credo mica! | I don't believe him at all!
Google translate doesn't translate mica because it is common only in spoken Italian while Google mainly uses books and written material to translate texts!