I'm guessing that your default language has changed.
(On the Tools menu, click Options. In the General settings section, verify that your default language is correct.)
(On the File menu, click Project Settings. In the General tab, verify that your default language is correct.)
We have seen this reported here a number of times with RoboHelp 8 and I have seen it myself. There's no real fix other than sitting back and waiting. The problem seems to go away albeit it might take quite a while. I suspect the fix comes with various Adobe updates as the dictionary is shared.
See www.grainge.org for RoboHelp and Authoring tips
Thank you, Absolutely_CHMing and Peter. I did check Project Settings, and found it set correctly, for US English. Upon this suggestion, I checked Tools>Options>General, and also found that set correctly.
The only thing I can think of is that I MAY have seen the Language setting in a Topic Properties dialog, and poked at it. However, I know I set it correctly, if I actually touched it at all. My current Topic and all the others I've checked at random are set to US English.
I kind of like the idea that it may just go away...hopefully soon.