Now supertuxkart can do:
Windows (fully test in win8 and win7 built with vs2015, mingw not tested):
Input characters using ime (CJK tested).
Fix crashes when try to paste unicode text into editbox in stk.
Linux:
Different keymap can be used in stk now, ie typing russian,hebrew (not bidi-aware) is now possible.
IME not supported.
Mac:
No work has been done, sorry mac fanboys/girls:( Though testing is welcome, esp when pasting words into stk.
Limitation:
No ime box is shown in fullscreen
(Windows)Only the used language in current setting of non-Unicode programs can be typed with its supported IME.