One might think the bilingual useful in the apprehension of a language, but usually they're not.
...A rude question in the wrong language....
The difficulty is probably most acute when one is a native speaker of the global lingua franca trying to learn a "smaller" language. Most people who learn English as adults probably do so because they must. It's probably not easy for them to appreciate one's desire to learn a language that is less-"important." Most people will appreciate the sentiment, but won't believe that you're serious. Some will prefer to believe that you're not, because that's easier.
People who do not speak English will appreciate any communication that you can manage. But the bilingual? Forget about them. Ask a well-placed question every now and then and be specific. Don't expect more than a competent answer, if you can get that.
The bilingual speaker will be unlikely to indulge conversation in the less-convenient of two languages. And if you do get a chance to practice with somebody who does not speak English, beware too much assistance from these same bilingual folks. They will almost always jump in with help that is no help at all.
As a resource when learning their language, the bilingual are pretty much useless, if not worse.