For some languages, complicated processing and feedback are needed as the user enters input. For instance, for the Asian languages, the user typically enters the pronunciation of the characters, then in a separate step, chooses the appropriate ideographic characters out of various possibilities for that pronunciation.
The input process is handled by a system called a input method. A simple input method within the X server is used to handle accent composition for European languages, but generally, input methods are separate processes that display feedback to the user, then send the finished input to the user.
The interaction with input methods is done using the X Input Method Extension, which comes standard with X11R6. There are input methods available for a number of languages, including Japanese, Chinese, and Korean.