Hi,
Look at these 3 files (example with french translation) :
/templates/rt_gantry_j15/en-GB.tpl_rt_gantry_j15.ini > save as > fr-FR.tpl_rt_gantry_j15.ini
/templates/rt_gantry_j15/admin/en-GB.tpl_rt_gantry_j15.ini >> fr-FR.tpl_rt_gantry_j15.ini
/administrator/language/en-GB/en-GB.tpl_rt_gantry_j15.ini >> fr-FR.tpl_rt_gantry_j15.ini
in the repertory /administrator/language/fr-FR/ (for the last one)