Some English words in the subject of an item are marked as incorrect by the spelling checker in Outlook 2016

This issue occurs because an incorrect language is used for spelling check in Microsoft Outlook 2016 in a multilanguage Windows configuration. To fix this issue, install the Description of the security update for Outlook 2016: June 13, 2017, and then follow the steps.

Important
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.

1. Exit all Microsoft Office applications.
2. Start Registry Editor:
   • In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
   • In Windows 8 or Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
   • In Windows 7 or Windows Vista, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
3. Locate and then select the following registry subkey:

   
   HKEY_LOCAL_MACHINE\Software\Microsoft\Office\16.0\Outlook\Servicing

4. On the Edit menu, point to New, and then click DWORD Value.
5. Type EnableLanguageFallbackInSpeller, and then press Enter.
6. In the Details pane, right-click EnableLanguageFallbackInSpeller, and then click Modify.
7. In the Value data box, type 1, and then click OK.
8. Exit Registry Editor.