Install the ClearPass Cumulative Patch 1 for 6.3.0, that was released today.
Unfortunately the planned Dutch translation pack did not make it in the 6.3.0 release, however it is added with today's Cummulative Patch 1.
After installation, you should have Dutch/Nederlands under Guest Home » Configuration » Translations » Translation Packs.
Please provide me feedback on missing or wrong translations, so we can improve.
Herman