Unfortunately for most of your questions the answer is "it depends." System requirements is more a factor of traffic and usage than application. A really processor and memory intensive will run just fine of minimal hardware if there's virtually no traffic, and the simplest of web sites can be brought down on a robust server if there's too much traffic for it to handle. That being said, in general complex applications with fairly heavy traffic will probably require a dual processor server. You'll want redundant hardware throughout where ever possible (such as power supplies or network cards) and you'll want hot swappable SCSI drives in a RAID-5 configuration and some kind of automated data backup (write to another drive or perhaps tape, depending on needs).
Making applications work is different languages requires scripting so that everything that appears as text comes out of a language file and that you have placeholders in the code that gets it from the database. For instance you want a color choice of white you would mark the code with something like {address} and when the script found that it would look in the language file for what to put there. If someone were set for German the script would look in the German language file for {address} indirizzo and would replace the marker with indirizzo. This takes minimal server resources to do, so performance isn't really an issue.
Basicly evrything you're asking, if on an adequate server isn't a big deal to accomplish.