When you make customizations to included language files their overwritten when you update SoluteDNS. To stop this you can use override files.
In the folder:
lang/override/ you need to create a file equal named to the main language file.
You can add custom language here. To override existing language values you can simply copy the language variable to the override file.
$_ADDONLANG['client_title_permission'] = "Access denied!";
$_ADDONLANG['client_title_permission'] = "Oops, you are not allowed to access the given zone.";