1. Twitter is much more mainstream in the US than in France and other European countries. One source I found shows the active users by country [1]. Using those numbers and assuming 320 million population for the US and 65 mil. for France one can see that 7.15% of US americans are active per month while only 3.3% of French are active per month. My interpretation is that the twitter users in France are much less representative than in the US. Also it is much easier for a very small but active community to break into the trending charts. Another hypothesis which I sadly could not find any numbers for is that the average twitter user in the US is posting MUCH more than the average french. Thus it is much more harder for fringe opinions to rise to the top trending.
2. Trending hashtags don't have to mean people have to endorse it. It could be a critical retweet or other kind of dismissal of racism. So just the fact that a racist hashtag is in the trending topic could also be interpreted as that people in france give a much larger response to racists tweets and try to express their dismay and thus circumstantially also push that hash tag. (I don't want to say that this actually happened)
But just these two points alone show why trending hashtags are a very weak indicator.
[1] http://www.mediabistro.com/alltwitter/twitter-countries-acti...