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).
