A stroke can definitely affect your speech, so some people may not pronounce their words as they did before. In some cases it might sound to others as if they have a "foreign accent", especially if they can no longer pronounce some sounds like "th" or "r" well, which are also sounds that non-English speakers have trouble with.

As for suddenly being able to speak a language they never knew? No. I have heard of some people becoming more artistic or creative, but that could be from damage to other parts of the brain that previously inhibited that behavior.