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.