My Mac AI CS4 spell check dictionary is set to Bulgarian. I don't know how this happened, but I can't seem to find a locale where I can change it back.
Preferences > Hyphenation shows English (USA) as being set.
Select your text and open the Character Palette. For pre existent texts, the dictionary takes the information of the language from this palette. Change it to what you need.
This is so usefull because you can have different languages in one block of text and check the spelling in one step. Click on the image to see it larger