There is a known issue here where word tries detecting the language as you type. This normally works fairly well, however sometimes word switches the language based on the input language. 

However, sometimes when editing a document in Office Word and suddenly all sorts of words are underlined as misspelled. Word talks about the French dictionary! 

To fix this issue in Microsoft Word where the Synonyms for a particular document is in different language or the proofing language/spell check is changed to French, Spanish, etc.

First select all the document (shortcut Ctrl + A) and navigate to 

Review Tab > Language > Set Proofing Language 

and then in the pop up , make sure the check-box 

"Detect language automatically" is unchecked.

Also ensure that your desired language is set as default (Select the language and click the button "Set as Default")