The reason for this is very simple: The Shareaza user community composes itself essentially from French users. You can have a look at the estimated composition of the network population on tis page:
http://crawler.trillinux.org/countries.html But plz note that these are only the Hubs, so the computers that coordinate the network, so the actual network population may differ a bit from the information displayed at the crawler page.
You should also know that if more of your friends are using Shareaza, the amount of content in your language will raise, too. For example you can copy the magnet links (URIs) of the files you are downloading and send it via instant messenger (MSN, ICQ, ...) to your friends and they can download exactly the same files, which will raise the total download speed for that file if you manage to get 4 or 5 people with a relatively high amount of upload speed to download the same file(s) than you.