One feels that the recently improved interface to create a localization, but here to update the localization of something missing.

Well, I like never translate new added lines in the interface Zabbix because they find very difficult - almost impossible. I am using for this TurtoiseMerge utility . I opened an old file and a new downloaded and easy to move only on the changes.
I mean, that if the interface on the locale edit page was added the navigation for added and a blank value, then virtually anything can be done directly in the interface without using other utility.

I think you can add a up and down buttons (centered always on top) to scroll the window and switch the input focus on the rows that are empty or filled in by default value (english usual). Thus, these lines can highlight a different color.