New pc needs a language that isnt in the first boot prompt

[ElStellino]

Hello,

I got today a laptop for a friend, and the core condition that I said in the shop before buying it, was: the laptop needs to run in English-UK, as she’s British and can’t speak Spanish (we live in Spain). Although they said "yes no problem, just select the language at the beginning, and the system will be in that language" – it isn’t as easy, as there is not English-UK. Just

català

español

euskara

galego

No trace of English.
Tomorrow the shop is closed as it is a festivity here in Spain, and wanted to solve this a.s.a.p.

As a side story related to this, we have at home my wife’s laptop, and that also had the OS in Spanish: although I tampered a bit and manage to set the system language to English, it has English menus but it still prompts messages in Spanish every now and then. It also upgraded to Windows 10 recently, and it didn’t solve anything. Now, for us it isn’t as important as we speak Spanish also, but our friend doesn’t, and having important messages appearing in an unknown language isn’t certainly cool.

How can I change the language without having to deal with Spanish anywhere in the system?

Thanks in advance!!!

Edit – just watch this video to see what I mean:
https://www.youtube.com/watch?v=uaCG1Oevq-k

Why at 6:46 does it show "password" and not "passwort"?
Why at 6:50 does it show "welcome" and not "willkommen"?

Edit 2: it seems just the log in-log out screens remain in the language of the first installation.
How to change these? Reinstalling the whole OS?