A LanguageTool add-in is available for MS Word 2016 or later in the Microsoft Add-on-Store AppSource. You can install it via Insert → Add-ins directly from MS Word.
The add-in works on Windows, Mac, and the online version of Word.
Select "Add-Ins" on the "Insert" tab:
Select "Store", search for "LanguageTool", and add it:
LanguageTool will then show up on the "Home" tab: