Currently, Squeak does not support Unicode. There is, however, some interest in Unicode and there are two attempts to do something.
As far as I can see, both attempts are unfinished and experimental.
Some years ago, Yoshiki Ohshima had a japanese version of Squeak, and recently he presented a proposal for Unicode support and a first implementation. Details are on this page: Multilingual Squeak
It would be a good next step to select the best from both proposals and to create an unified implementation proposal. Selection of a common implementation policy should also examine the question of possible or desireable changes in the VM. A lot of problems are still not solved. For example, we do not currently have the possibility to use one of the IME-s (Input Method Editors) to enter text in Unicode.