I don't know too much about this, having used only english on the pda, but when developing applications for pocket pc's unicode is the norm. Embedded Visual C++ has built in functions for unicode as well as conversion functions to single byte text.
The author of PHM Registry Editor seems to have a unicode capable notepad utility in PHM Pocket PC Power Toys.
Here is the link...
PHM Pocket PC PowerToys
Here is also a tip from someone regarding the display of Japanese/Chinese on pda's.
Reading Chinese/Japanese on Pocket PC
The steps involved should be the same for any language you should need to edit/display.