Mac OS X 10.7.3 came out recently. Among the many little things it corrected here and there, it also brought new localizations for Catalan, Croatian, Greek, Hebrew, Romanian, Slovak, Thai, and Ukrainian.
This means that the System now provides proper spelling dictionaries for these languages too!
Since Outlook tape directly into the standard Mac OS X spelling API, it should provide spell checking for these languages as well automatically.
(though Hebrew remains an issue since there is no proper right-to-left support in any of the Office for Mac applications)