For typical western 101/103 keyboards that lack the kor/eng toggle key found on korean 102/104 keyboards, RightAlt is used instead for this purpose. There is another key for hanja conversion, which happens to collide with RightCtrl - the host key for vbox by default - though the vbox host can be reconfigured to another keystroke so this is no big deal.
The real problem I am having is, depending on the version of vbox client / guest addition I have installed on my machine, the guest fails to assign RightAlt for kore/eng toggle. Two interesting tidbits that I've noticed:
1) toggling thru RightAlt works just fine with XP installed fresh out of the box. Installing guest addition seems to break it right away.
2) Strangely, the guests built from the same xp images never had this issue on linux (ubuntu) hosts.
The very first thing I've tried was changing the keyboard identifier in the registry with couple other changes here and there. This is known to work well in most cases with misbehaving toggle keys, but no luck there.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\i8042prt\Parameters]
"PollingIterations"=dword:00002ee0
"PollingIterationsMaximum"=dword:00002ee0
"ResendIterations"=dword:00000003
"LayerDriver JPN"="kbd101.dll"
"LayerDriver KOR"="kbd101a.dll"
"OverrideKeyboardType"=dword:00000008
"OverrideKeyboardSubtype"=dword:00000003
"OverrideKeyboardIdentifier"="kbd101c.dll"
"KeyboardFailedReset"=dword:00000000
Then I have discovered the two keys in question (kor/eng toggle and hanja conversion) have scancodes (in hex): 72 00 and 71 00, respectively. (Technically, their make codes are F2 and F1, but whoever assigned these codes was silly enough to use numbers in the region where programs would normally expect a break key, and these two keys don't even have break codes to begin with
So in practice people just treat them as 72 00 and 71 00 instead lest they should confuse whatever poor program that happens to take the input). Tried mapping RightAlt (38 E0) and RightCtrl (1D E0) to the above two via scancode map in the guest's registry, and this didn't work either. As the last resort, I sent keyboardputscancode command to the guest thru vboxmanage:vboxmanage controlvm guestname keyboardputscancode 72 00
the output looked like it should (scancode[0] 0x72 scancode[1] 0x00), but the guest was still registering RightAlt only (as opposed to kor/eng it should have been). That pretty much leaves me out of options, which is what brings me here to ask you guys for some help. I am guessing this somehow has to do with whatever the guest addition (or the windows version of it anyway) does to the way vbox passes scan code for user input to the guest OS. Just a wild guess though.
Oh, the korean IME on the guest OS isn't broken, I could still use the mouse pointer to click on the kor/eng button on the language bar GUI. It's just a huge PITA to take your hand off the keyboard when you have to type mixed sentences that alternate between the two languages.