OnKeyPress events

Hello.
I'm trying to write a Script to substitute keys pressed while having a text box focused with other keys - my idea is to enable writing in my native language with a us keyboard.
My problem is that i have no idea how to actually substitute the keys pressed - what i figured as an idea was to register OnKeyPress events and depending on the key pressed to substitute it with the correct one. Problem is, that i can't think of any efficient way of telling the browser not to write the letter that was pressed - anyone having any ideas?

Keep a seperate Javascript variable where you keep the string with the translated keys.
After each keypress, add the translated key to the variable, and then REPLACE the entire text-box value with the value of the variable.

Behaviour may differ on a per-browser basis though, as some browsers alter the text box before the keypress event, and some do it before.

Alternatively, you could simply add a second, read-only box, where the translated text is shown, and leave the original input as-is.