Unicode supports 150+ scripts which can be used to write thousands of languages. No matter how good your Unicode support, there is probably some obscure script for which you do the wrong thing. But, in the real world, most software (whether proprietary or open source) supports a limited set of languages, and you test it works correctly for all of them, and every now and again you might add support for a new language (due to business requirements, or, in the case of open source, because someone volunteers to do it)-at which point any issues specific to that language and its script are found and fixed.
The unicode consortium maintains open source software to handle a lot of hard cases (not just case folding but canonicalization, formatting, line breaking...). Doesn't magically make all your problems go away but takes away a lot of hard work that you're most likely to get wrong.
