New Outlook Editor uses wrong language for Spell Check

From your outlook web go to new message Home>New Message open the new message as if you want to compose a new message. go to Option>Editor>Editor Setting> click on Manage Spellcheck language>Add another language, Select your desired language as a proofing language> click on Add, click on the three dots ... beside the language you would like to remove and click delete. click done, and your preffered language is saved. This worked for me.