Thanks for your response and concern. Really appreciated.
Languages file of CB Builder are not located in the language directory of Joomla and therefore language substitution cannot be use to add any missing.
I have downloaded and expander the original FR language zip file available on CB site : plug_language_french_fr-FR_2.4+build.2019.02.22.zip
It contents a lot of files and directories amongst which the is an extension/cb_plugins directory.
This directory contents a second zip file with same name : 023_plug_language_french_fr-FR_2.4+build.2019.02.22.zip as the original FR language zip file available on CB site : plug_language_french_fr-FR_2.4+build.2019.02.22.zip
Expanding again this second file gives a file named admin_language.php a cbplugin directory with all separated language files for CBplugin also php and not ini files.
I do not known how it works and how CB determine which "admin_language.php" it should use according to selected language.
This admin_language.php contains the strings and the French translation. As I do not knows how it works, I cannot add the missing string, if I can discover its name, in the file. Moreover, it will be overwritten at upgrades.
When you install CB plugin "plug_language_french_fr-FR_2.4+build.2019.02.22.zip" it says that it is 100% translated, that's why I think some strings are missing. This is an issue for the development team, not only for the translators.
Kind Regards