I lived in Houston and there are some really dumb people. Especially down town. If you had asked for a walmart or an HEB you would instantly have been given the proper directions to go. There are also a bunch of brits in Houston for the oil industry. I know I played on a company soccer team league there for several years. Was playing against Exxon, subsea7, and a few others where most of the teams seemed to be brit imports. If they weren't brits they were Venezuelans.
But usually most people in America know exactly what a supermarket is even if you ask just for that. Or you can just ask for a grocery store as well. Or the specific name of a place you are looking for. In Houston I think the major grocery chains are HEB, Walmart, Krogers, and Fiesta Mart. There are a few others, but those 4 are just about everywhere in Houston.