Currently the plugin loads the localization files for de, en, fr and nl. However each loaded file seems to overwrite the previous one which results in the language always being Dutch.
In addition this localization seems to always overwrite the localization of jQuery-UI's datepicker component which is included with DokuWiki and correctly localized already.
The immediate fix seems to be to not load the local files at all. This at least displays the jQuery UI datepicker correctly localized again.