This definition ( http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_230 ) states:
3.230 Name In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set. The first character of a name is not a digit. This document has a table of the characters included in the portable character set: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap06.html#tag_06_01 Which is a subset of the unicode alphanumerics. So no, it does not mandate arbitrary unicode alphabetics. Only the ones listed there.