Here is what I got from http://dev.day.com/docs/en/cq/current/wcm/using_rich_text_editor.html#Spelling%20Checker:
"The spelling checker will operate in the language of the website by taking either the language property of the subtree or extracting the language from the URL; i.e. the en branch will be checked for english, the de branch for german."
Dose this mean the spell checker plugin will work for different language, based on the domain of page URL? For example, on page abc.com/page1.html it will use american english to do the check and and on page abc.uk/page1.html it will use British english? I can't config the language used by the spell checker plugin, right?
Yes it works on different language. Refer  how the language is selected and dictionaries details.
Thx Sham. So the language is selected based on the language branch subtree, and if there is no language branch and en_US will be used. But what if there is no langugae branch but the user want to use languages other than en_US? Is there any configuration can be used? or there is any other workaround?
AFAIK not aware of such option.