Here is the way to enable China offline maps on Windows 10 Mobile v1703

[chantszhim]

According to https://msdn.microsoft.com/en-us/library/windows/hardware/mt131465(v=vs.85).aspx, we need to use Interop Tools to change the "ChinaVariant" value of "HKEY_LOCAL_MACHINE\System\Maps" in registry from 0 to 1 in order to enable HERE Maps data and offline maps for China, Hong Kong and Macau in Windows 10 Mobile version 1703.

Actually Hong Kong and Macau is not part of Mainland China and does not require this setting in previous builds. Don't understand why HERE Maps treat them as part of the China maps category now.