Schwabian German accent is very difficult to understand. Actually visitors should be aware of:
Schwabian German is notoriously difficult to understand and Germans from outside the region frequently moan that Schwabians add an unnecessary 'schw' sound to many words. The beleaguered Stuttgart manager, Giovanni Trapattoni, has famously struggled with the language and the local media ridicule his German as 'Trapattoni Deutsch'.
Instead of immediately asking if someone speaks english, try, if you can manage it, to learn at least a few phrases in German. It was always much appreciated when I gave an effort, and most of the time, everyone speaks several languages and will gladly help out.