To stop being polite about this

Hallelujah!!!! The Op has got it!

You’re in a battle of your own making that you’re bp ever going to win. You’ve presumably lived in this other country long enough to know that you can’t meaningfully compare it to the one you were raised in? We all do it after the honeymoon period but it’s useless unless you truly think you can change a cultural mindset. Just keep replying in the home language if it’s that important to you.

But you don't 'belong' there. You are clearly different and not local. You can use the local language and they can speak English to you. If you go in those shops regularly they will know that you won't switch to English and that you are entirely fluent in the local language. You aren't going to become short and dark and accentless so it's up to you really. You won't ever belong in a 'born-there' way because you obviously weren't.

This will be an OP who is pissed off about quite a bit in life

Yes

I suspect the op must have very few interactions at work or with local friends for this to be such a big deal for her

Come again?

what on earth are you on about?

So someone not fluent wouldn’t be “equal member of the community” to…. You? Simply because they’re not fluent?

Maybe you’re not as good as you think you are at the local language.

Yes, sorry OP, but that's how you come across. Either that, or you are desperate to be something you are very obviously not. Or, they don't agree with your assessment of your fluency. Or, you are prone to being constantly offended or annoyed by people.
Maybe you are assuming some of them are natives based on how they look??

Agreed. The only times I’ve ever been upset about being treated like a foreigner in three decades of living away was when there was an unpleasant assumption or stereotype at play.

If your life genuinely is full with speaking in the language to colleagues and friends - then it is a littler odd that the odd retail exchange so upsets you

You say they're trying to show off their English, so are you not trying to show off your grasp of their language, too? Perhaps your accuracy isn't as good as you think it is.

Sometimes you might know the technical aspects of the language but fall short on slang and local dialect, it sticks out like a sore thumb, and they probably think they're making life easier for you.

I really don't know why it bothers you.

OP, I'm sorry to say you sound arrogant, and like you have a chip on your shoulder. Perhaps your Spanish (as that seems to be the assumption) isn't as good as you think it is, especially your pronunciation. Perhaps the locals want to practice their English - you're in THEIR country, so you could be kind and let them, instead of moaning about feeling "othered" while in the same breath stereotyping the entire nation and bragging about your superior aryan looks. Tall and blonde versus short and dark - you sound straight out of an Enid Blyton novel! You do know there are plenty of tall blonde Spaniards, don't you? Especially if you're in Catalonia, which was not influenced by the Arab conquest.

I suggest you educate yourself a little, beyond polishing your language skills like some trophy. And talking of language skills, if you're living in Catalonia and want to be treated like a local, make the effort to learn some Catalan. That they will appreciate.

This comes with kindness but firmness from someone who has spent 40 years living in a different country, and genuinely understands what it is to embrace being a foreigner with humility and respect for the country where she has made her home.

If you tell them you're Danish, do they still carry on speaking to you in English??

Or, straight to the point, "If I wanted to speak English, I'd still be living in the UK."

I grew up in Denmark lived there until I was 20. When I go back to Denmark if a shop assistant hears me spewking English with dh or the childre wven if I direct them in Danish thats accent free. They try to steer the conversation to English.

I have not worked oht hoe to deal with it bar at times I say (in Danish) we can continue this in Danish. Doesnt always work.

I think it's a bit presumptuous to start talking to a stranger in the language of the country you THINK they're from without any indication from them.

If they're conversing fluently with you in the language of the place you're both living in, and you decide to respond to them in X language because that where you think they're from, I think you run the risk that they may think you're debasing their language skills and feel they need a translation.

Occasionally I meet people in the UK who come from the country I lived in as a child (they tend to retain a strong accent however fluent their English may be). Depending on the relationship, dynamic and how well we're getting on, I might ask which part of X they come from, and relate that I lived in X as a youngster. I wouldn't just start chattering away to people I don't know in X language without so much as a by your leave or giving any context.

They love to speak English. They are just doing it because they can and they enjoy the opportunity. I'd just carry on replying in their language, not yours.

FWIW I used to find this slightly frustrating at times when my French was better than their English, but ultimately I just took it for what it was - a show of kindness

Are you always so uptight.

Jeez, it’s a compliment that they want to go it!!

It may be nothing to to with what people think/assume about you, just people wanting to use/practise their English. Just accept it and get on with your life - or switch to internet shopping.