Mart,
We have Office 2003 English with some MUI (Dutch, French, German, ...).
I used the Custom Installation Wizard to create an MST file.
We did not use a GPO to distribute the package.
I also installed the "Compatibility pack for the 2007 Office system" on some computers now (not all of them).
I don't think I have seen the problem yet. At least not on my computer.