It is possible to change the default system-wide language in Windows 10. For example: you bought a laptop that displays system text in Spanish, but would like to use it in English.
To begin, click the start button and then click Settings:
Click Time and Language from the settings panel:
Now, select Region and Language on the sidebar, followed by the Add a language button:
Then select the language you would like to use from the list (it is possible to search for it) and click next once finished:
You may be prompted to install speech and handwriting features on the next screen (leave these checked if required). Be sure to leave the "set as my default Windows display language" checked and then click Install:
At this point, you have changed the system language for your account (on your next sign-in) - but the sign-in screen and other user accounts will still be set to the previous option. To fix this, scroll down in the settings window and select Administrative language settings:
Then select the Copy Settings button under the "welcome screen and new user accounts" label.
On the next popup, make sure that the two checkboxes under "copy your current settings to" are checked. Click OK once done:
Now, reboot your PC and you should be up and running in your new language!