Alternatively - use Control Panel/Add language to install & enable English (US) - took ages, required reboot, showed error message (failure configuring Windows) but finally worked. (NB. Before rebooting, ensure you know how to enter any special characters in your login password on a US English keyboard.)
Then with US English enabled, Windows8-RT-KB917607-x64.msu installs and you get a more up to date version of winhlp32.exe.
Is Windows8-RT-KB917607-x64.msu enabled only for users of English (US)? It looks like Microsoft's quality control has seriously slipped with Windows 8, particularly with updates.