The instructions below will allow you to run the update by editing the pre-install script to bypass the check for previous versions of the files. (The alternative is to do a complete reinstall of MS Office -- to get back the language-specific files that you removed -- and then reapply the updates. That would certainly be the safer course, but I haven't seen any problems after having followed the below instructions on my Mac with OS X 10.5.4).
Here's what I did:
- Copy the file Office 2008 12.1.2 Update.mpkg from the mounted DMG to some other location -- e.g. to your Desktop.
- Use Finder's Action (gear) menu or contextual menu to Show Package Contents on the above copy of the .mpkg file. Navigate to the Contents » Resources folder, where you should see a file named package_updatable. It is probably listed as a Unix executable file.
- Open this file in your favourite text editor (e.g. Text Wrangler) by using the File » Open menu item in that text editor application, or by dragging the file on top of the text editor's icon in the Dock. You will see that this file is a Python script. Look for the lines in the script that look like this:
Change the break to be continue:if not os.path.exists(file_path): break
Then save your changes.if not os.path.exists(file_path): continue
[robg adds: I haven't tested this one.]

