After installing a languagepack help contents should be available when you create a site collection in that language. Sometimes however SharePoint gives you a message that the files have not been installed when you click the help button or go to the Help-settings page for your sitecollection.
You can force SharePoint to install the files using the hcinstal.exe tool in the bin folder under the SharePoint root (c:\program files\common files\microsoft shared\web server extensions\14\bin):
hcinstal.exe /act InstallAllHCs /loc 1043
This command will install all available help content for LCID 1043 (Dutch)
After this running this command (which can take up to 10-15 minutes easily) you need to run the SharePoint Products and Configuration Wizard on each front-end server.
I think you can also use the following 2 PowerShell commands, but I wasn’t able to test these after the first solution worked: