Internet Explorer 9 Language Packs are provisioned by different x86 and x64 Windows Update files, named as
IE9-Windows6.1-LanguagePack-xAA-LLL.msu, where AA = architecture (86 or 64) and LLL = language code (e.g. nld for Dutch).
When trying to install these language packs via
wusa.exe, Windows Installer might throw the follow error:
Windows update "Update for Windows (KB982861)" could not be installed because of error 2147944003 "Fatal error during installation." (Command line: ""C:\Windows\system32\wusa.exe" "X:\IE9-Windows6.1-LanguagePack-x86-nld.msu" ")
Search no more. Although there doesn’t seem to be any resource (for now) that states so formally, you cannot deploy IE9 LPs unless the same Windows 7 Language Pack has been installed previously.
In other words, after installing the Windows 7 Dutch Language Pack,
IE9-Windows6.1-LanguagePack-x86-nld.msu for Internet Explorer 9 installs succesfully too.