The easiest way to avoid character encoding issues is to create an .epub with
Sigil. Simply open Sigil and copy and paste your text in Book View (WYSIWYG) mode into the default document (Section0001.xhtml). If the French accented characters are displayed correctly, press F8 to enter the name of the author and the book title also change the language from English to French.
(Optional: Highlight each heading and press CTRL+3. Then click the Generate TOC from headings button.)
Save the .epub file and open it with
Kindle Previewer to generate a .mobi file.