I don't know where the French language attribute came from, but it may have popped in before you unchecked "Detect language." Something even more subtle is if the box is checked to not check spelling or grammar. If you don't know the underlying structure, it feels very much like a gremlin or virus.

As a start, for this document,

1. Press Ctrl+A to select all text in the body of your document
2. Then go to the Reviewing > Language setting and make sure that the language is set to English [US].

Just as if you were using a colored font, copying and pasting will copy the language attribute. Starting typing at the end of a word set in a language will continue that language.

If you have styles set to automatically update from your formatting, they can pick this up and make it part of the style definition. Text copied in from another Word document can carry the formatting as well, includiing in styles.

I hope that has not happened. If you still have problems, feel free to write back.