For an overview of language packs and how to install them, please see the customizing page.
Until the official Luna Translation Center is finished, which will allow for translating the software online, language packs have to be created by hand.
To start translating Luna, you need software like Poedit to load the translation files and start translating.
The stopwords.txt file contains a list of words which should not be indexed in users comments. This should be used to filter ter out common words which do not add value to a search, for example, in English, words such as "the" and "it's". For more information, check Wikipedia.
Until this will be possible online through the Translation Center, language packs will have to be packaged and uploaded manually. You can just comment a thread announcing and linking to your language pack.