So you're trying to change the string? You don't edit the files directly. You use the language overrides feature shown below.
www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived
Language overrides are per-language plugin. So if all you have is the default language then for example you'd only add an override to it.
As for the language plugins other than Default we just package them from transifex. We don't officially provide translations. They are an entirely community driven effort. So if you're missing language strings or out of date you may need to add a language override as needed. In future releases when we convert our plugins to standalone Joomla extensions we'll be providing official translations for some languages.