The only sure-fire way to replicate the behaviour:
- Remove ~/.config/sublime-text-2 (to make sure it's not a plugin or configuration issue)
- Launch Sublime
- Type something in English
- Switch to Greek via keyboard shortcut. (Switching via the panel indicator appears to work.)
- Type something in Greek
- Press Ctrl+Z. Undo will not work.
I think that Sublime and Ubuntu and are not coordinating about the language switch.