Dynamic gravity form translation in front ?

Hi,
I LOVE your plugin but I have an issue. I explain :
I have a dynamic French & English website and I would like to use a dynamic dual - language form.
When users selects the french version, forms are in french...
And when users selects the english version, forms are in english...
In my WP theme, I use a classical process like this <?php _e('label', 'theme_name'); ?> but I don't understand if I can do the same thing with your plugin ?
In which files I have to place this code ?
I didn't find the .po files to edit my labels. Where can I find them ?
I have to change 'theme_name' by what ?
Thank you very much for your answers and sorry for my little english.
Sincerely,
Grégory