In Sublime Text 3, unicode characters are not shown correctly. In gedit it works fine. I tried saving the document as UTF-8, Tried setting default encoding to UTF-8 but stil
Sublime doesn't support Complex Text Layout at all I'm afraid. That means you don't even get proper Arabic or Hebrew, let alone Sinhala.
Hopefully this will be added some day but it's not a straightforward feature. As Sublime is focused on code rather than natural language it might not be amongst the author's priorities.