There are many forms to translate this sentence. Some of them formal, and also informal ways.
According to the original word
If you don't speak Spanish in a Mexican neighborhood, you're screwed.
It can be translated a Mexican way like this
(informal) Si no hablas español en un barrio mexicano, ya valió.
(informal) Si no hablas español en un barrio mexicano, ya valiste.
(informal) Si no hablas español en un barrio mexicano, ya valió queso.
(informal) Si no hablas español en un barrio mexicano, ya valió "burger".
(informal) Si no hablas español en un barrio mexicano, te ching*ste.
Those ways you can translate it, however, is important to note that there are no the unique forms to translate it. The mexican culture is rich of informal language and curious words.
According to your original translation
Si no se habla español en un barrio mexicano, ya valió queso.
The subject is "you" at the original sentence, so the translation to the spanish must be at second person too, at your translation, you're modifying the subject to "Si no se habla", that seems to be an impersonal form, that is not partially correct, first of all because of the sentence coherence. I’d recommend to translate the sentence at the same gramatical form as the original sentence.
On this words, we're using an spanish concept called tacit subject that refers to a sentence where the subject is assumed as "you" (in this case). You can assume the subject with the sentence context too.
Si no hablas
The subject refers to "you", as in the original translation
If you don't speak
At the translation, the subject is referring to tú, you can note that adding tú to all sentences, the coherence will not be disturbed.
Si tú no hablas inglés
In this case, we're not using tacit subject, so the subject is clearly visible there.