This is a very interesting thread! :)
We, at Greenpeace, are working on an ACS classic 4.2 site. So we are using Oracle as the RDBMS (this solves most of the problems when dealing with utf-8 and multiple languages, since interMedia -in theory- should be able to index multilingual content and you should be able to perform complex search based on languages -if supported by Oracle-). We are using Henry's and Rob's patches to AOLServer and ACS and they work mostly fine.
As pointed already, you should take care in the part were you send information to a client (browser) or recieve from one.
The question you are asking, if I understand correctly, is "how do I display multiple languages in one page?". I would say that the only way to do that is to send and recieve in utf-8 but in that case the browser needs to support it and the user need to have installed the correct fonts. But do you really see pages having discussions in different languages/alphabets in the same page?
I am not 100% sure that you can't display multiple charsets in one page but if you are sending back to the browsers one charset in the HTTP headers, then you might run into trouble (Henry do you know what would happen in this case?)
Just my 2 cents.
