Strangely, I can't recreate the error either within my original document nor in a cut-down version I was going to upload. I have been checked and unchecked various boxes though, so may the act of that has triggered Word to refresh a preference or something...

In the original document, it appeared to reset the proofing language on opening, but even that hasn't manahed to recreate it.

If it does again however, I will upload a copy. Until then though, we might have to say it is resolved...