To stop being polite about this

I agree with this. They are obviously just trying to be friendly to you.

You have a chip on your shoulder. I worked with a few Polish people and there was one woman who insisted on speaking to our colleagues in Polish, thus leaving me out. Everyone of them answered her in English as i was part of the team. My point, stick with the native language and answer them in that. They will soon pick up your drift. You won't feel like a foreigner, won't need to be rude, and they can go practise their English elsewhere on the net. They are probably just trying to show off but it is not meant with ill intent, it is meant with a friendly touch.

Being 'reminded that you don't belong' in a city more typically takes the form of hostility. Refusal of service on a poor excuse. Bad service. Racial slurs, bricks through your windows, attacks and assaults, stereotyping you as the cause of all ill in the society. That kind of thing.
It sounds like the local population are treating you with a courtesy that many people 'foreign' in a city would be extremely grateful for.

That's certainly not been my experience and I speak passable French.

They want to practise their English, that’s all.

I am guessing the OP hasn’t carved out much of a life for herself in terms of work colleagues and friends.

Because if you worked - then your day would be spent conversing with colleagues in the language.

If you had made friends - then you would converse with them in the country’s language

So most of your days would be full of conversing with people in the language. So the odd shop assistance assuming English would be water off a duck’s back.

The fact the op so exercised by it - indicates she doesn’t get much opportunity on a day to day level to converse closely with people in the language

Op is completely fluent

do you work op? Have colleagues? Made friends?

Goodness OP, you must be unhappy about quite a lot in life

This used to drive me crazy when I was living as an au pair to learn a language. Almost impossible to get anyone to reply to me in their language but I so wanted the practice! I was actually very good at speaking it but that didn’t seem to matter so totally get how frustrating it is.

You clearly don’t look like a native so it’s not unreasonable for the locals to think you are not a native. Therefore, they are probably trying to be polite by speaking to you in a language you may both have in common.
Do you live in an area that is not very diverse and so majority look “native” ?

It can be frustrating OP, but just roll with it. I was in your situation when I lived abroad, fluent in local language, and yes, people practised their English on me. Generally I didn't mind. Sometimes it led to interesting conversations.

You know, English (and I do mean English, rather than British) people are often the same (and with less reason) and often make it quite clear that they think they are better than wherever you're from as well. It deosn't make how you're being treated ok, but it might give you cause to reflect that it's at least not as bad as it could be.

I would assume that they are detecting that you are not native from your accent and are excited to be able to maybe practice English or trying to make you feel welcome. As someone that has moved to another European country with a difficult language to master, I would find this heartwarming, I wouldn't be offended. In my experience, if someone is trying to belittle you because you are different or be racist, they would make it more difficult to interact with you (even if they could speak English perfectly).

You’re trying to fight against reality. From what you say, you will always stand out as foreign because you look and sound foreign. This is not going to change so you might as well accept it.

People speak English to you because they think they are being polite and helpful. If you don’t like it, just say (in the local language) "I’d rather we speak in Xlanguage". That’s all you can do.

Do you actually 100% feel that you belong there OP or do you still feel like a bit of an imposter. You are letting their kindness and good manners irk you when you could just smile and say please let me practise my Japanese or Italian or Portuguese or whatever.
It is what it is and if people aren’t being racist or unkind then it’s nothing to get upset about. You just have to be patient and go with it, - with good heart.

I voted yabu because

A. English is a lingua Franca, people speak it in so many countries and it's a useful language to have. Of course they want to practice it whenever there's a chance.

B. You would be very unreasonable to stop being polite to shop assistants who are just trying to give you good service. Especially if there is food service involved.

I too think you’re being over sensitive. Accents cause interest/curiosity, even within the UK. Even within England. We are northerners. Our grandsons live in London. Their accents are sometimes a topic of conversation here. Our 4 year old asked why we pronounced ‘bath’ and ‘jump’ differently on his last visit. People notice differences. That’s life.

Can you just look at them blankly and say you don’t speak English in the local language?

my friend is going to Japan soon to learn Japanese and this is one of his worries, he wants to learn Japanese and speak it but he knows that soon as they look at him they will try to speak English. I told him to instead say in Japanese sorry I am German or something 😂. Assuming that not many people in Japan would speak German.

What would happen if you smiled and said, in the foreign language, ‘is it ok if we continue speaking in xxx as I love using your language?’ Do you think they would object?

Could you not use the local language to thank them for trying and say ‘it’s not necessary, we can do this in Spanish/Korean/Whatever’?

You have to look at their intention which is to be helpful. I would see it as accepting/welcoming. Not an insult. YABU

Maybe they think they’re being helpful? Maybe they want to practice their English? Maybe you’ve just told them that their cat is shagging their mother rather than asking the price of pasta, who knows 🤷‍♀️

There’s loads of reasons they might do it. Don’t get annoyed.

You could see it as a contribution you are making to your city.
the city has embraced you, ad you have built a life there. In return, you can offer some. It’s citizens a little English light conversation when they ask for it.