The descriptions will be in whatever language the author posted them in. It would seem that the plugins you happen to be interested in were written by a native French speaker who has not provided an English description - it is quite common for the plugin description to be in the native language of the author with just a note indicating whether the plugin supports other client languages.
Google translate can be helpful to translate the page if you aren't sure of the capabilities (including support for the EN client) of the plugin.