Mostly Dutch, some English
Deciding on the language is a perennial dilemma: it's fun to meet with and get input from fellow Perl users from around the world and if it's all in Dutch quite a bit of the presentations might be lost on them.
But since a large part of Perl events and online activities already take place in English, we'd still like this to be one place where Dutch speakers can give and listen to presentations in their native language.
So speak Dutch, if you can...
Therefore our approach is that at least all native speakers of Dutch and those who can comfortably express themselves in Dutch are encouraged to do so. Other speakers are of course fully welcome to use English.