Why are random words underlined Office Word spell check?
Word indeed shows lots of spelling errors on your French text:
The reason is that the text is encoded in UTF-8 and not in ANSI. Your document is encoded as ANSI, but it contains UTF-8 characters. Word somehow detects the presence of UTF-8 and displays the characters correctly.
For example, the word that you see as "Enfin" is not 5 characters, it is only 4 characters. The third character is U+FB01 LATIN SMALL LIGATURE FI that takes up two bytes and looks like: "fi".
This text looks like someone is playing a joke, using Unicode.