hello
impossible to have kdenlive in French
version 0.9.8
switch application : only English
my computer : mint 17.2 kde

Hello,
I already replied onfrench forum that 0.9.8 missed all translations (sorry for the mistake),
you can download any kdenlive-data_0.9.10 (eg here) and install them (via "sudo dpkg -i kdenlive-data_0.9.10*.deb") and you should get translations.
Or you could have searched french mint forum which explains how to use PPA to upgrade to 0.9.10 on your mint version.