Sometimes you want to use exisiting translations for locales, which do not directly match.
An example would be a website targeting Germany (de_de), Austria (de_at) and Switzerland (de_ch, fr_ch, it_ch). Although you may have
exisiting translations for German (de), French (fr) and Italian (it), there are problems using it directly.
When using themes you want to changes some of the translations specifically to the theme while keeping the standard translations for most of the text.
Duplicating the main translation file is a headache for maintainance.
This how to provides a solution.