I gave up on MDT 2010 for installing the language pack and went back to a script using 'lpksetup.exe /i * /p "%~dp0nl-nl" /r /s' for installing the language pack and an other using 'control intl.cpl,, /f:"%~dp0Dutch.xml"' for setting the language settings, both work perfectly. If anybody wants to know how to set the options in SCCM 2007 to make this work, just let me know.