On the po plugin's page it is clearly stated that "when the plugin has just been enabled, or when a page has just been declared as being translatable, the needed POT and PO files are created". Yet on all my attempts, only the pot file was created. Do I have to create the po files manually somehow?

To be precise, these are the settings I put in my wiki's setup file to enable the po plugin:

add_plugins => [qw{... po ...}],
po_master_language => 'de|Deutsch',
po_slave_languages => 'en|English',
po_translatable_pages => "mytranslatedpage",
po_link_to => 'current',

… followed by "ikiwiki --setup mysetupfile".