Besides compiling postgres with unicode support also make sure that your oacs database is actually unicode encoded, e.g. with createdb -E unicode yourdbname. (psql -l lists all databases and their encoding).
The following is rather speculative because I never tried that myself: if you need specific sort orders in postgres then take care to set the locale while doing initdb (initdb is called right after installation, so this cannot be changed later without destroying all your databases). E.g. after installation do a "LC_ALL=de_DE initdb".
Allegedly this slows down postgres quite a bit, and I wonder if it works with OpenACS at all. There were some issues with tree_sortkey requiring sort order of LOCALE=C, but I guess since this is not character based anymore it is not a problem anymore (Don?).
For multilingual sites there is the acs-lang package, which provides message catalog functionality. It is far from finished (lots of UI missing), but since there is a lot of interest here this is propably going to be improved sooner or later.
Date formatting is often hard coded in the toolkit with the plpgsql to_char() function (in postgres). I am going to grep for the most commonly used formats and will suggest to write some centrally defined functions instead (really soon).