Kmail version 1.10.1 (SuSE 11.0) spell check defaults to German. There does not appear to be a way to set this to default to another language. This is a nuisance, requiring users to select the spell check menu, and select one’s appropriate language, for every email sent with spell checking.
I had the same problem. I was able to change it via the control panel under “Regional & Language”. You probably have to restart kmail, don’t remember.
Still if you need to change the spell checker back and forth between two languages like me, there is no easy way do doing that 
I set the regional & language to regional to US, and left the language blank. The only language setting for this is German. My guess is this is something that will be resolved in the 11.1 release.
Hmmmm. In that screen there are three option groups on the left (KDE 4.1.2 in English):
- Country/Region & Language
- Keyboard Layout
- Spell Checker
I was referring to “Spell Checker” and it looks like you were changing “Country/Region & Language”. If it is not there try searching for “spell” in the search bar. I don’t remember if I had to add the English language but I added Portuguese through YaST/Software Management. There is a very useful filter there: “Languages”. 