Here is a picture of all of the characters available: http://www.jazz2online.com/J2Ov2/dow...hp?fileID=9102 (note that it is very tall, so it might be hard to see zoomed out).

It doesn't have everything (for example a lot of polish letters are missing), even though it has really weird letters like ð and þ which are hardly used by anything. Still, it would be possible to hack JJ2 to get it to recognise more keyboard inputs and at least be able to type what is available.

One interesting thing to note is there is a lot of empty space. Conceivably, you could probably add in a whole bunch of new characters to the sprite file and then tell JJ2 to display them properly, but there would be problems with doing that, and I don't know what would happen if two players had different versions (does the game give an error?)