You have my full sympathy NoTart. I'm a translator living in Germany and I speak fluent German yet I constantly get spoken to in English by people who want to use me to improve their English. It often has nothing to do with politeness whatsoever and, as you rightly say, there are circumstances where it is totally unnatural.
Over the years, I have developed a number of techniques for actively discouraging it. One is to reply in German no matter what, another to mumble or reply in rapid, colloquial, obfuscatory English, another to correct every single mistake the other person makes, another to exclaim "Hey, I'm not at work! Let's speak German!". With more thick-skinned types, I have also, on occasion, been known to intimate that it offends against my (British!) sense of what is polite and makes me feel like an outsider rather than being welcoming.
When my daughter is there I always insist that the opposite party speaks German, if need be by politely explaining first that we are using the "one person-one language" method with her and that it is confusing for her to have a German-speaker speak English to her, second that it is vital to her integration that she gets treated just like the German kids (and she in any case also speaks fluent - three-year-old - German), and, third, that it is critical that she only hears absolutely correct English pronunciation and grammar so that she grows up with English at mothertongue-speaker level .... Actually, she is quite a star in the whole business. She can identify a non-native English speaker from their intonation immediately and simply refuses to speak anything other than German with them .
Yes, I sound like a right old cow. But it riles me! And I swear I will shoot the next person who asks me for English lessons!
(Actually, to be fair, I do voluntarily help some people. But these are those who don't expect it!)
And breathe ....