You should go to Settings > Accounts > Sync your settings and turn off the Language preferences option.
- 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.“
- On the Administrative tab, under the „Welcome screen and new user accounts“, click the Copy settings button.
- Check the Welcome screen and system accounts and the New user accounts options.
- Click OK.
- Click OK again.
- Reboot your computer and upon restart, you should see the new default language globally set through the operating system.