I experimented a local solution that is not a full satisfaction, but works for me as a customization.
I'm not sure this is adressing really perfromance as it is not much more than inserting some variables in the string as we often do.
I added a global $GLOBSTRING array in the game, globalized in get_string(), so very global key terms could be systematically adressed.
ex : $string['createnewcourse'] = 'Create new '.$GLOBSTRING['course'];
The global $GLOBSTRING array is loaded once within a wellknown language string that we are sure that it be loaded soon enough (# !!) (I used local/lang/xx_utf8/local.php, but having to adress very soon in config a get_string there to be sure.... (ugly).
Work exactly as expected, that is, allowing to have a fast, 10 minutes work shift of the overall semantics of Moodle !! Exactly what we need for our customer adaptation....
Cheers and thanks to all for your thoughts.