I can´t speak English is translated as "eu não sei falar inglês/eu não falo muito bem inglês". So, I think in something academical, a test, an exam, the expected answer would be "can´t".
Although the "I don´t speak" would be a valid answer, that would come in second to me.
Plus, I wouldn´t let a question like this be on my test, unless it was on some exercise immediatelly under some topic - like CAN vs MAKE, for example. Precisely because of this ambiguity.