There are countries where either persisting in the local language or asking nicely, will meet with a negative response. Take the Netherlands, for example, where 90% of the population is said to speak English now. A foreign novice–intermediate learner trying to practice Dutch in everyday situations is likely to be told “Let’s just speak English and not waste time.” Your attempting to persist with your less-than-fluent command of the local language is interpreted by your interlocutor as disrespect for his time. If you want to learn the local language, you are expected to place the burden on your language teacher, not random shopkeepers etc.

I'll admit that I've had very few opportunities to travel abroad thus far, but, for what it's worth, my experience has been the same. In fact, most people seem to be a bit gladdened to find that a foreigner (perhaps especially an English-speaking tourist) is making an effort to learn their native tongue. And also, likely, a bit amused by my shortcomings—but that's no big deal: it's how you learn.