Hi!
1. Serendipity scales like a "usual" php Application. That means, the speed heavily relies on how well your server operates, what hardware it has, if you run reverse proxies, database master/slave setup, if you're running PHP opcode caches, if you run PHP as module or CGI, if you use apache or lighthttpd etc.
That said, s9y should typically operate at around 6 to 50 requests/second. There's no limit to users or blogs, I've seen blogs with hundreds of authors, and the SQL queries should be as fast with 10.000 authors as with 1.000. Millions of authors might introduce database-row-sorting issues, that can be fixed with query caching etc.
So: It scales.
2. Can multiple users make posts to a single blog?
Yes, that's the main focus of Serendipity.
3. Does Serendipity fully support muliple languages such as French, German, Spanish, Japanese, Chinese and Korean?
If you run the blog in UTF-8, that is possible. s9y has a multilingual plugin that allows you to translate a single entry into multiple languages. It works for many people, but for some people it has a different workflow than they'd expect. Depends on your usage scenario.
If you just want to have multiple blog entries (independently) written in different langauges and be displayed on the same page, this is no problem in UTF-8.
The only thing were you should consider a different blog engine is when you want to make people sign up and have their very own, independent blogs. That works with s9y using "shared installation" mode, but requires some server-side setup and is a bit awkward to use if you expect true single blogs.
Serendipity has means to "isolate" authors and categories into individual looking blogs with a single installation, but depending on your usage scenario that might also not be what you'r elooking for. The only "true multi-blog" system I know of is MoveableType, but of course it lacks in other areas
Best regards,
Garvin