You need to decide whether your package joins a translation project or whether you want to connect to translators in an ad-hoc way.
In a translation project, teams of translators work on a number of packages. They work together by exchanging common practice, technical dictionaries, or advice. Common translation projects are:
Connecting to translators in an ad-hoc way may be useful in the beginning if your package has highly specialized terms and translator volunteers already waiting. In the long term, though, you will get more language coverage by joining a translation team.
When and how are the files in the po/ directory created, is related to this choice. See section Put the PO Files under Version Control or not? for details.