I just downloaded a copy of the wiki-module (http://moodle.org/download.php/modules/wiki.zip).
I didn't find any language file...
nl version is included in the latest language pack at http://moodle.org/download/lang/
I have to point on the fact that there is an nl language-pack, but it is under developement. For the wiki, I still have a small list of ajustements and untranslated helpfiles.
Same as Koen for french language : available in last language pack, but in development !