This is short tutorial on the steps to be followed for changing the proofing language for a Word document in Office 365. In order to specify proofing language, launch a Word document then navigate to the Spelling section.
Click on the down arrow on the Spelling icon and select Set Proofing Language from the list.
This would display the following Language selection dialog box.
Now select your preferred proofing language and click on the OK button to save the changes.