I met a similar problemnow, in 2010:
My PC & Office 2007 are both configured to Czech language. Recently I got a
document written in Czech whereas the language in it was (incorrectly) set to
Slovak. To avoid ridiculous spell-checking I changed the language to Czech,
expecting that all becomes OK by itself. But the red underlining did not
disappear. When I manually started spell checking, the "Slovak language" came
back, instead. After that, I was seeking and found this post I am just
replying to. I was ready to delete the Registry key, as Chris was advising,
but I found no such key in the Registry.
To continue my effort I heuristically tried to change the document format
from 2003 to 2007, checked & unchecked the Compatibility mode, but it brought
no result.
Evidently, there still is an error of this kind in Word 2007.
Does anybody know a solution?
This is not the only shortcoming concernig spell-checking I have met in Word
2007. I was used to provide some specific text strings (which in fact were
not words of the language but abbreviations etc.) by the "do not spell-check"
attribute to avoid red underlining. I didn't find this facility in Word 2007
any more. When I want to get rid of useless and incorrect red underlining, I
must do things like create a new style with this attribute and apply it, but
to simply select a piece of text regardless of paragraph boundaries and
assign "do not spell-check" works no more.