I've done research about that.
Maybe you already know about that, i've compared German and French jar files and did some test : the problem does'nt come from the translations ConfluenceActionSupport-properties files (where you modify the translations), nor from the other files (there is just a line "groupId=com.atlassian.confluence.plugins" on the pom-xml files on the french pack that doesn't appear on the german one, but i deleted it and it doesn't seem to affet the system).
So the last difference between the two is that there is a "ç" in the word "français". I think that causes the problem. I'm not sure however, but if you just try to change 'français' in "Francais" (with a "F") it could work ? And i don't think a "c" instead of a "ç" stand such for an inconvenient, regarding the handicap of not having the french traductions at all. Don't hesitate to keep us up to date, i'm following the discussion with interest. Thanks again.
John - we hope to upgrade the hosted version this week. Please keep your eye on http://jira.atlassian.com/browse/HS-58 for progress updates.