I suspect you are exactly right. Your browser doesn't have the correct character set enabled to display the text properly.
Most people leave the CWP default language at English because most browsers have that enabled, and most technical people at least read english. When a browser connects to the captive web portal, it supplies the languages (character sets) that are enabled on that browser, which is what our CWP keys off of. If the browser suggests that a non-english language is preferred, and we have language enabled in the CWP, then that's the particular web pages that will be served up. If there's no match whatsoever, then we fall back to serving up the default page.
So, since you enabled only French, and set the default to French, when any browser connects they will get the french text but will render it via whatever character set the browser has.
yes, I understand.
But I do not have this problem on other web pages between English and French.
My browsers interpret well accents principle. I think rather a problem of coding your web pages (header, charset ...).