How to apply the jrcore_lang strings change from Ninja update to my custom skin?
Using Jamroom
Here is an example of what those changes are: (screenshot)
If you've updated your cloned skin to also used the language file, then yes, you'll need to copy the extra items over to the /skins/yourskin/lang/en-us.php
But if you're not using any language other than english I would be tempted to go the exact opposite way and replace all the language file entries with their actual string and ignore the LANGUAGE tab all together.