This one is a bug, to be honest. My surname is French, and I prefer to write it out correctly with the accent grave on the first "e". Every page on my site would then include at least two accented letters, because my name shows up twice in the footer. On some pages my name shows up two or three more times.
Normally, this wouldn't be an issue. But on some pages the presence of the accented characters is enough to cause AdSense to display non-relevant ads in French. This happens whether the browser indicates a preference for French or not. When I reported this to Google, this is the answer they gave me:
Hello Eric,
Thank you for bringing this issue to our attention.
We are currently working as quickly as we can to address this problem. As soon as we have more information for you, we will email you again.
We appreciate your patience.
Sincerely,
The Google Team
Until this is resolved, I've decided to strip out all accents except on the pages that are actually in French.
Tip #4: Check your keyword density
Although Google doesn't release exact details as to how they determine the ads to serve on a given page, they do tell us that it's the text content of the page that matters, not the meta tags. Before serving ads on a page, then, you might want to check its keyword density. A good, free tool for doing this is found here: http://www.ranks.nl/tools/spider.html
This lets you fine-tune the page before exposing it to the AdSense crawler.