So far the Watch Dogs series has dared not stray from the United States, but that may change if evidence gleaned from an updated ending to Watch Dogs 2 is anything to go by. Since this story relates to the ending of Watch Dogs 2, there are big (but not massive) spoilers below.
Ubisoft rolled out an update for the game recently, adding a bunch of new content and, as you've probably gathered, an addendum to the game's original ending. That ending comes in the form of a phone conversation, discussing how hacker group DedSec is "going global". The Middle East, South America and Europe is mentioned as headquarters for this expansion, though the most telling sign is a map coordinate displayed.
That map coordinate, 51.462014, -0.112504, happens to hit slap bang in the middle of Brixton, London. It's hardly a confirmation, but Ubisoft is known for sprinkling its games with hints of what's to come, and indeed, if DedSec is going global (ie, anywhere outside of the US), then it'd make sense for the game to go global too.
Here's the new ending by the way, in case you'd prefer not to boot the game up:
