This has nothing to do with the framework. Javascript supports unicode characters, and so anything built on top of it for displaying strings will too.
The reason 280 Slides doesn't do this correctly is because it implements its own text editor by intercepting all keypresses. Unfortunately, there's not enough information in the JavaScript key event mechanism to properly handle internationalization this way. We'll eventually be removing this custom text editor in favor of a more standard approach.
Separately, while there's not currently anything built in for localization in Cappuccino, there will eventually be the same system that exists in Cocoa, which makes it trivial to do "i18n" with little or no actual coding.
However, if you copy and paste from another application you'll see the underlying framework handles it fine.