Window 10 is not bound to the language you have used for installation. You can change the Window 10 system language for every user account on your PC.
1. Adding keyboard / spelling languages to Windows 10
Use your administrative account to sign in to Windows 10. Click on Start Menu and tap on Settings.
Select Time & Language in Settings window.
From the left sidebar, choose Region & Language then click Add Language from the right pane.
Find the language you are going to add. You may notice that languages are sorted by alphabetical order regarding to the default Windows 10 language. If the language has some local variations, you may click your desired language.
2. Installing display / system languages to Windows 10 via language packs
After the language has been added to the list, you can to download and install the Language Packs for changing the Windows 10 system language. You will find the download clicking on the language and then on Options.
To get the Windows 10 system language pack, click Download. Take into account that it might take a few minutes since downloading and installing a Language Package is a lengthy process!
3. Changing the Windows 10 display / system languages of a user account
Select your language if you want to change your Windows 10 user account’s language and click on Set as default. Then you need to sign out from your user account and sign in back again to apply the changes which it makes you to see your new Windows 10 system display language.
Example: Changing Windows 10 system language from English to Catalan
To see how to change the Windows 10 display language for an Catalan user account, let’s do the following steps.
First change the default language from English to Català. In that case you need to click on, Start Menu, tap on Settings, select Time & Language, then Region & Language on the left sidebar. On the right sidebar, tap on the Add a Language, scroll down to Català and select it by clicking on it.
After having downloaded the Catalan Language Pack it will show up in the list of available Windows 10 languages with the note Language pack installed.
To change the Windows 10 display language for the current user, you just need to select Català as the new language and then click on Set as default.
Windows 10 will show you that Català language will be display language after next sign-in.
You just have to sign out from your Windows 10 user account and sign in back in to see Catalan as the new display language for nearly all Windows 10 system elements and menus.
4. Changing the display language of welcome screen, login screen, sign-in, sign out, shutdown
Changing the user account display language in Windows 10 does not change the language setting of what happens before logging into that account. If you want to also change the language of welcome screen, login screen, sign-in, sign-out and shutdown, another step is necessary.
Open the Control Panel with a right click on the Windows 10 Start button and navigate to Clock, Language and Region. Then click on Region.
In Region, switch to the Administrative Tab and click on Copy settings.
This opens the Welcome screen and new user accounts settings. Click on the checkbox Welcome screen and system accounts to apply your new system language also there. When you also want to define that language as default for new Windows 10 user accounts, check also New user accounts.
After a restart, Windows 10 will use the changed language also for the welcome screen, login screen, sign-in, sign-out and shutdown screen.
Changed language of Windows 10 welcome screen
Changed language of Windows 10 logon screen
5. Uninstalling Language Packs in Windows 10
Open the Run dialog by pressing Window + R and type the command Lpksetup /u to open the install or uninstall display languages dialog. Select the language you want to remove and click on Next. Uninstalling a display language might take some minutes. You will not able to uninstall your main Windows 10 system language.