Why did Microsoft Word switch to French spell check on me?

"To fix issue like this 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"")"

"There are two things at work here probably:

Word tries detecting the language as you type. This works fairly well when you got like 5 to ten words minimum and eventually provides you with correct spell-checking even in multi-lingual documents.
Word switches the language based on the input language. I have Polish installed as input language, along with the proper keyboard layout and when I switch to that in Word I get spell-checking in the correct language.

The easiest immediate fix is probably to select all, then click the language in the status bar and select English.

It might be that your default style is set to French (for whichever reason; that usually doesn't happen automatically); you may want to set the language there too."