Starting with LEPTON 4.3.0 you can use custom language files.
Simply copy a core language file of your choice, rename the file to ( for example) EN_custom.php and you are done.
The custom file will automatically replace the standard core language file.
Keep in mind, that you have to be aware if core language files are modified via update of LEPTON. You can check the changelog or the update package for modifications.
Notice: the same procedure is working with addon and template/theme language files!