> being able to mix arbitrary languages into a single String object
Unless I missed something that is impossible with Unicode. Mixing multiple languages would require a way to specify the language used for case conversion, sorting and font rendering settings mid string and I don't think that Unicode has that. For example try to write a program that correctly uppercase a single string containing both an English i and a Turkish i in your favorite Unicode supporting language, the code point is the same for both, and you generally only get to specify one language globally or per function call.
You can write a string with words from multiple languages, you just can't easily modify it with operations like case conversion. But sorting shouldn't depend on the origin language anyway, it depends on the language of the reader. All words in an English dictionary are sorted in "English" order
Displaying is also questionable. If you want japanese/chinese/etc. rendered correctly in your browser you have to mark corresponding sections with a language tag, they have different rules on how several abstract graphemes shared between them should be rendered (amount and shape of strokes).
Unless I missed something that is impossible with Unicode. Mixing multiple languages would require a way to specify the language used for case conversion, sorting and font rendering settings mid string and I don't think that Unicode has that. For example try to write a program that correctly uppercase a single string containing both an English i and a Turkish i in your favorite Unicode supporting language, the code point is the same for both, and you generally only get to specify one language globally or per function call.