OK, for the 50/60Hz switch: there's a big chip on the board labelled 315-5246 (with pins in a "zig zag" formation). Lift pin 57 of this chip (i.e. desolder it from the board or just cut it) and solder a wire to it.
Connect the other end of this wire to the middle pin of a SPDT toggle switch, and connect the outer pins of the switch to +5v and ground points, just as you do for the MD and Saturn 50/60Hz mods. (Pins "O" (+5v) and "C" (ground) of the 7805 voltage regulator are suitable.) Putting the switch in the +5v position selects 50Hz mode, Ground selects 60Hz mode.
As for a language switch, I'm not sure of the "correct" way to do it, but here's a quick hack: find the chip labelled 315-5237 and solder a switch between pins 23 and 24. Leaving the switch open selects "English" mode, closing it selects "Japanese" mode. It's evidently not the proper way to do it though, as closing the switch causes some peripherals (such as the Light Phaser) to stop working.
If you're interested in other mods, such as RGB output, have a look here:
I made the mod on a french master system II and it's even easier ... no need to remove pin 57, just find where this pin is connected (follow the track by returning the board).
When you get the point, you can see that this point is located between 2 pins (marked PAL and NTSC ... thanks you mr. sega !) and that it's connected to the left PIN (the one marked PAL) through a simple wire !
Just cut this wire and connect the middle pin to the middle part of the switch and the others 2 parts to each one of the pins described before (the "PAL" and "NTSC" which are in fact +5V and GND) and ... that's it ... no more difficult than cutting a wire
So I think that the best solution if people are looking for a SMS 2 unit is to search on Ebay France (www.ebay.fr). You will get a "easily modifying" SMS 2 with the quality of RGB output (yes, the french SMS 2 have a RGB output and no RF output)
Hope that all this stuff will help new or future SMS2 owner ...
btw mike, thanks a lot for all the mods informations (genny, sms ...) you made available for us ... since a long time now