Change spell check language in the Windows 10 mail app for whole email

Solution 1:

It is important to first set the language Windows key + Space bar before start writing or answering a message. Mid-message language changes are not possible

Solution 2:

I recently bought a new laptop and because the model is new it wasnt available in my country, or at least not with the good specs. So I bought it via Amazon from Germany and had to change the language. I followed this "how to" and it seems it worked fine!

How to change the language globally on your PC

Before we dive into this guide, it's important to note that if you're using a Microsoft Account, this preference will sync across devices. If you only want to change settings on one particular computer, you should go to Settings > Accounts > Sync your settings and turn off the Language preferences option.

1. Use the Windows key + I keyboard shortcut to open the Settings app.
2. Click Time & language.
3. Click Region & language.
4. Under Languages, click Add a language.
5. Click the language you want to add, and select the specific variation if applicable.
6. Select the new language and click Options (and download any the language packs as necessary).
7. On Region & language, select the new language, and click Set as default.
8. If applicable, under Country or region, select your location to enable the Windows 10 to give you local content.
9. Use the Windows key + X keyboard shortcut to open the Power User menu and select Control Panel.
10. Click Add a language.
11. Click the Advanced settings link in the left pane.
12. Click the "Apply language settings to the Welcome screen, system accounts, and new user accounts."
13. On the Administrative tab, under the "Welcome screen and new user accounts", click the Copy settings button.
14. Check the Welcome screen and system accounts and the New user accounts options.
15. Click OK.
16. Click OK again.
17. Reboot your computer and upon restart, you should see the new default language globally set through the operating system.

You'll find this feature especially useful when you work on a mixed-language environment where you need to configure workstations with different preferences to accommodate users. In addition, this feature will help in those times when you happen to purchase a new computer that ships from another country with a different language, or even when you move to another region of the world and you want to change your PC preferences to match the local settings.

Source: http://www.windowscentral.com/how-properly-change-system-default-language-windows-10

Maybe it can work for you to. Make sure to change the whole system language to English for it to work. Maybe you do not want to do this but its at least one way of getting it to work.

Solution 3:

You can change the language of an e-mail by changing the keyboard language. If you have multiple keyboard languages set up, you can switch with Windows key + Space bar.

I figured this out after reading this answer: https://superuser.com/a/1038844/31930.