For multi-lingual readers, dictionary support is one area where the Sony shines, I think. I like to read books in French on my PRS-650, and I have been generally happy with the results when I have needed to use the dictionary (which is often). It is SO much easier to just tap on the word than thumb through the dictionary every time! The two built-in English dictionaries are fairly good as well.
In comparision, I think only the Kindle offers additional dictionaries, and their selection seems quite limited:
http://www.amazon.com/gp/bestsellers...inc_2_6_last#1
Judging by the reviews, none of the French dictionaries listed are very good, and despite Kindle's strong (albeit US Domestic) sales, they don't seem to support a very wide language choice either, and anything other than English you have to pay for.
The shorter answer to your question is I don't think you'll find a better electronic French dictionary than the one on there.