1. Add the Dutch translation provided on patch ticket 1816739.
2. Patch for bug 933526 needs to be applied to resolve Group By issue (needs to be confirmed first, but I have complete confidence in Jonathan Marsden who researched this and wrote the patch.
3. As pointed out by Jonathan, there appears to be a lot of missing string in various message bundles. At a minimum, all keys should be present in all bundles, even if English strings are used.
4. Contact all the people listed in the credits that did translations and see if they are willing to update the bundles. If not, at least #3 above will ensure English versions are present at least (any chance exceptions are thrown if a string is found missing anywhere? Not sure, but better to avoid the possibility entirely by ensuring all keys are present).