Sadly, this doesn't appear to actually work on 10.5; Font Book does everything the hint says it will do (quit-restart of the app also required on 10.5), but the dynamic language/script-specific collection only contains Helvetica and Times for everything I've tried (including Arabic, which is not supported by Helvetica or Times).

It's a shame this feature is so buggy on 10.5, because it really is a neat feature. Thanks to porg for the hint; I look forward to using it on later OS versions ;)

It doesn't seem to really work in 10.7 either.
It shows only some core set of fonts which have a support for the given language, bu t it doesn't fit to quickly sort all of the available fonts.