Hello everyone. Just bought a new laptop running Windows 11, originally it came in Polish language. I followed all the tutorials I could find and easily changed display language, input language and most of the system's language.
Windows display language and system language are two totally different things. Windows system language is the language of Windows core, the same as the language used in install media. Display language is whatever user chooses to use, by downloading a language pack and setting it as his / her display language.
Changing Windows 11 display language has absolutely nothing to do with what edition you are running. You either can't do it (Home Single Language Edition), or can change it (all other editions).
Display language can be set individually for each user account. For instance, my typical Windows setup has my main account in British English, with additional user accounts in both my native languages Finnish (Suomi) and Svenska (Swedish), as well as in language of my adopted home country German (Deutsch):
When I need to assist someone using one of these languages, I can simply switch user to get for instance a screenshot in correct language.
Display language can be any of the official Windows languages, but
it does not change the Windows system language!
The purpose of a language pack is to change the most essential, most used Windows UI terms, notifications, dialogs and so on, to language the user prefers. However, some Windows core stuff will remain in system language.
To completely change the system language, the easiest way is to just to reinstall (clean install) Windows in your preferred language. As Windows digital license is not language specific, you could download Windows 11 ISO in your preferred language, and clean install the same Windows edition as your currently activated Polish Windows. It would be automatically activated.
However, with clean install, you would lose all your settings, files and installed software.
If you want to change the system language, and keep all your accounts, setting, files and apps, you must use
Windows System Preparation Tool (Sysprep.exe). Here's how:
1.) Install all language packs you'd prefer
2.) Run Sysprep.exe from folder C:\Windows\System32\Sysprep.
3.) Accept all defaults, and press Enter, or click OK:

4.) Sysprep will now run, and Windows will reboot to OOBE:

5.) After restart, you can now select a new system language. OOBE will ask, in which language you want to continue. Unfortunately, this screen is quite badly designed, giving not a lot of information. You just need to take my word for it, just select the language you'd prefer to have as system language, and click Yes:

6.) OOBE will now run in your selected language, as if this was the first boot after installing Windows. Please notice, all default Windows requirements apply. You must be connected to Internet, and you must by default use a Microsoft account, when OOBE is run. Because all accounts, settings, files, and apps are intact, when OOBE asks you to setup the initial user account, you can't use the same MS account email than in original installation. The same applies to if you have used a workaround to setup Windows with a local account, it cannot be used in OOBE now.
I am always installing Windows 11 with a local account. Running OOBE again, I cannot use my original account name Kari. I will use a dummy account Dummy, then when finally back on Windows desktop, I will sign out from it, sign in with my original Kari account, and delete the Dummy account.
If setting up with a Microsoft, you must use another MS account, then when on desktop, sign out, sign in with your real, original MS account, and delete the second account used for OOBE.
That's it. Your Windows system language has now been changed. Some details, like device descriptions in Device Manager and such, might still be in original system language, but as much as is possible, will be changed to new system language.
Multilingual Windows user,
Kari