How to load the language file fr-FR.ini oren-GB.ini found there:
JPATH_ADMINISTRATOR/language/en-GB/en-GB.ini
JPATH_ADMINISTRATOR/language/fr-FR/fr-FR.ini
or
JPATH_SITE/language/en-GB/en-GB.ini
JPATH_SITE/language/fr-FR/fr-FR.ini
In your extensions and force usage of thoses files, in order to, for example, display correctly translated buttons in a toolbar (the same that is in admin panel on Joomla! frontend).For those of you who are interested, here is the code the force the loading of global language coming from Joomla! backend:

	// Force loading of backend global language file
$language = Factory::getLanguage();
$language->load('joomla', JPATH_ADMINISTRATOR);