If you have the Home Single Language version of Windows, you cannot change the Windows UI language, the only way to do that would be to backup all your files, then create a bootable Windows 11 USB, boot your device form that and clean install Windows in the correct language, it is a big job.
With regards to activation, if there is a pre-installed Windows 11 licence on the device, that is stored on the motherboard and Windows will activate automatically after it is reinstalled.
This process will not work with a device that was pre-installed with the Windows 11 Home Chinese or Korean versions, those versions are very specific and have different types of licence.
You will need and empty 8GB USB flash drive.
Click this link: https://www.microsoft.com/software-download/windows11
to download the Media Creation Tool (Click on Download Tool Now), with that you can create a bootable USB flash drive (min 8GB) using that tool.
Then, Boot your PC from the Installation Media you just created to begin installing Windows 11.
During the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 11 on this PC'.