Enables/Disables Unicode keyboard translation.
To obtain the character codes corresponding to received keyboard events, Unicode translation must first be turned on using this function. The translation incurs a slight overhead for each keyboard event and is therefore disabled by default. For each subsequently received key down event, the unicode member of the SDL_keysym structure will then contain the corresponding character code, or zero for keysyms that do not correspond to any character code.
A value of 1 for enable enables Unicode translation; 0 disables it, and -1 leaves it unchanged (useful for querying the current translation mode).
Note that only key press events will be translated, not release events.