The lookup table is in libraries/Keyboard/src/Keyboard.cpp. The lookup table is '_asciimap'. The bottom 7 bits of each byte are the USB key code and the top bit is a flag saying whether the SHIFT key should be down.
Note that there is no place to put a flag for the AltGr shift so you can only type the two characters on the left side of each key.
You can find the USB key codes here:
http://www.usb.org/developers/hidpage/Hut1_12v2.pdf