At the moment you still have to enter the language and time details on every boot. On the real thing the cpu keeps running all the time at a very low frequency. The only way to do this in mess would be to play these drivers using savestates. I still have to finish the last couple of things to fully support savestates.
What already should work is boot the system, set details, power off and power on again. Between power off and power on it should be possible to change the mounted cartridge image. I haven't been able to test this fully yet though; it might be that there is some code in place which forces the emulated systems to restart when you change a mounted cartridge image.