You: hej må jeg have... et varm chokolade?
Them: of course, will there be anything else?
You: hvis det er okay jeg ville gerne øve mig på dansk *smile* åbenbart har jeg virkelig brug for det
Them: selvfølgelig! det kommer til... fem-og-halvfems kroner. Sidder i ind, eller ude?
If you don't explicitly say that you are trying to practise then yeah, they are going to default to whatever makes the interaction go smoother. Everyone does that, all the time! But people are generally pretty supportive of another person trying to learn. And of course, people aren't going to assume you think they can't speak english if you say you're practicing (it's not hard to smile and say how lovely you think the language is or whatever.
In general waitstaff and service workers deal with people being indecisive, slow, speaking with confusing dialects, speech impediments all day long. It is not an imposition to work through an imperfect language - and its pretty much the only way anyone ever gets good at a language.
I also get the impulse to be a little jealous of those natural born sunny extroverts who find it easy to do this. For me it does require pushing through a little discomfort, and often I wouldn't if I were busy doing something else. But it's 100% my own discomfort, not the other person. Being asked to hold a conversation in your own native language is not a chore