Additional language string proper location

I understand about language specific plugins and the language override plugin and those are working as explained.

But where is the best place to put NEW language strings (for example, for newly created fields). Do we put those in the language override file, even though they aren't technically overriding anything? Or do we put those in the language specific language file (for example, ja-jp/language.php), with the fear that they will be destroyed the next time that the language plugin is updated?

To avoid losing the strings I recommend using the language override plugin. We've just discussed override usage and will be implementing official override support in a later release by allowing you to edit the language plugin from CB > Plugin Management and specifying overrides directly in the plugins backend without having to edit any files.