UMA will work anywhere you can get a WiFi signal that meets speed and latency requirements and will let you connect to T-Mobile servers. This should absolutely work from both Korea and Japan.
The phone should work in both Korea and Japan, and you will probably get 3G coverage as well. Looks like Korean GSM providers use the 2100mhz spectrum and Japanese GSM carriers use both 2100 and 1700mhz. The T-Mobile version of the 9700 supports both of these bands.
I'm not sure T-Mobile has roaming agreements in these countries however. Looks like from the following link they do, but I'm sure it won't be cheap.
GSM Coverage Maps | United States | T-Mobile USA, Inc | Roaming Partners