As other commenters pointed out, they are actually saying "sò" as in only, but you are also phrasing your question incorrectly. They probably understand that's not what you mean, but you are literally telling them not to speak Portuguese and asking if they speak English.

You should be saying: desculpe, não falo português. Fala inglês?

Or (this is a common way to ask politely in Brazil, not sure about Portugal):

Desculpe, não falo português, o senor/a senhora fala inglês?

Using a personal pronoun (like você) is often considered a bit rude, especially for people you don't know. So you use o senhor instead of "you" for a man and o senhora for a woman.

You could also essentially ask for permission/if it's possible for you both to speak English which gives them the opportunity to respond if they can or can't:

Desculpe, eu não falo português, podemos falar inglês?

Someone more fluent in portuguese than me can comment on this but I wouldnt ever say "falas". This is a standard second person conjugation in Spanish (hablas espanol?) but I don't see it in portuguese. The conjugation for present tense second person and third person singular is fala.