I have a Japanese version of Microsoft Office 2007 Standard, upgraded from Microsoft Office 2003 Standard.
Is it possible to install an English Language Pack or English Language Interface Pack?
I've never done it, but it seems like it's possible.
So it looks like you can add languages to any base/starting Office installation.
Office language packs are apparently less limited than Windows language packs. The later can only be added to an English version, but Office is not so limited.