Hello MDL community! I am not sure if this is known here already. A search yielded no results. If you need additional interface languages AND/OR spell check languages AND happen to have the CD/iso from that language, you can install them the following way without having to obtain Microsoft's MUI packs: Install your version of Office 2010, for example the English language version. Then insert the office 2010 cd of the language you want to install in addition (German in this case). The installer will then recognize the installed version and offer a language option. Select German and then customize the installer to include German language proofing. The installation does not require another key or activation and the additional space consumed is marginal. After installation start word and go into review > language > language preferences. There you can configure editing, display and help languages between the installed ones. Another great feature I have just discovered there as well: You can delete "English U.S." from the list of proofing languages to prevent office from defaulting to it every time you really want proper UK English .
Apparently this does not work the other way around (first installing the German Version and then starting the English one simply wants to install the English one).