Getting back on my own post, proof that lacking of documentation is sometimes hard.
So for people still on 1.6 and @MrEduar
For common translations : (no module or admin)
$_LANG['name_of_your_tpl_file_MD5_of_the_translation_needed'] = 'Your translation';
with : for example
- name_of_your_tpl_file : the name of your tpl without the .tpl
- MD5_of_the_translation_needed : https://www.md5hashgenerator.com/
Which is gonna give you : (example for string 'Text #' and tpl : shopping-cart.ajax.main.tpl)
$_LANG['shopping-cart.ajax.main_2fb3b950fd7711136f7f251ae5fbdbdc'] = 'Texte n°';
Do not forget the underscore between the name of the tpl and the MD5 hash.
Note : the file to update can be the main translation file or the one in your theme folder (ie : themes/theme_name/lang/fr.php) (change the iso code)