At barenplatz and Marktgasse. It's a street vender in the outdoor market. Soft shelled taco's at fantastic. 3 for 12 CHF. Closest to mexican food I have seen in europe so far...
the Torta Carne is also very very good and big. They speak english well... usually a bit of a line up but worth the wait.
They also sell specialty mexican items: Mole sauce, homemade wheat and corn tortillas, tortilla chips (the good ones), other mexican sauces... refried beans...
do they take lunch-checks ?
Edit: nevermind.. it isn't likely given they are an outside vendor