Define better solution.

Using file.csv means you have to import it every time.

Using file.po it will be installed automatically everytime you install the module (if you place the file in <module>/i18n/ ). I have run into the issue that sometimes the old translations are not correctly replaced, so that is an issue to keep an eye on. But that also can happen with file.csv.

Given the above information, I would use file.po.