replacing key strokes

i was looking to develop an keyboard for multilingual application. where i will require to capture the keystrokes at global level and replace them accordingly. now using WH_KEYBOARD, i am able to hook onto the keystrokes pressed globally. The issue being is i cant replace the keystroke. rather when i interpret it and try to send another key, the old one pressed goes along it. EG: i want to replace 'A' with 'Z', and i pressed 'A', it will print 'AZ' rather then AZ. So do u have some good solution for it as of how to work on replacing it.