Setting the Proofing Language in Office Word
OneDrive - Word online
The language setting are in the individual document and can not be changed for all documents in the OneDrive. The language will remain the same as the language setting of the document when placed into OneDrive.
In word online and desktop version, under REVIEW, you can "set proofing language", but be careful where you are in the document as you can accidentally change different words (paragraphs, etc) to different languages.
In the Desktop version there are more setting to make the whole document be the same. Unchecking the "detect language automatically" will help.
In word online, you need to select all text (Ctrl+A) and then "set proofing language" for the whole document to be set to the one language. If you make sure that the document has saved before closing it, the language setting will remain on the document.