Use the Windows key + I keyboard shortcut to open the Settings app.
Click Time & language.
Click Region & language.
Under Languages, click Add a language.
Click the language you want to add, and select the specific variation if applicable.
Select the new language and click Options (and download any the language packs as necessary).
On Region & language, select the new language, and click Set as default.
If applicable, under Country or region, select your location to enable the Windows 10 to give you local content.
Use the Windows key + X keyboard shortcut to open the Power User menu and select Control Panel.
Click Add a language.
Click the Advanced settings link in the left pane.
Click the "Apply language settings to the Welcome screen, system accounts, and new user accounts."
Reboot your computer and upon restart, you should see the new default language globally set through the operating system.