German proofing tools might not be included in the language version of
Word that you have. For example, the U.S.-English version of Word 2003
includes proofing tools for English, Spanish, and French, but not
German. If you have Word 2003, see
http://www.microsoft.com/office/editions/prodinfo/language/proofingtools.mspx
for more information about how to add proofing tools in other languages
to Word 2003.
If you do have German proofing tools installed, then you can get this
error message if Word thinks your text is in a language for which you do
not have proofing tools installed. To fix the problem, open the
document where this problem occurs, press Ctrl+A to select all of the
text, and click on Tools | Language | Set Language | <choose your
language> | uncheck the "Detect language automatically" and "Do not
check spelling or grammar" boxes | Default | OK. Then press F7 to rerun
spellcheck.