Internationalization wrong filenames in languages

It took me quit a while to find out why my language is not used in the theme. After changing the filename in the /languages folder from nl-NL.po to nl_NL.po and the same for the nl-NL.mo to nl_NL.mo, the international translation is loaded.
In fact this seems to be faulty for all translations!
All ‘-‘ should be ‘_’ to comply with the WordPress conventions.