Actually all recent windows version should have these characters installed. I am actually running a english windows and neither did I have any problem with most languages except Indian Hindu.
No, not fully unicode. The text strings are code page based. Double byte characters are represented by a special lead byte followed by a second character. So double byte characters are included in a "single byte" string.
So the new version will be 100% compatible with Patch Y, except people in the old version will see random looking Latin-1 characters if they see some double byte text.