O.k. I've narrowed it down to a Joomla problem rather than a CB/CBsubs one. Turns out that no matter the default language setting in Joomla it would run English. Now I haven't figured out what's causing this behaviour yet but the work around is to make the english language files inaccessible or to uninstall the english language all together.
It's not pretty or the desirable way of doing things but for now I got things up and running properly in Dutch.
Thanks for hanging out with me in solving this. If I ever find the culprit I'll let you guys know!