Just an idea; after the landing of bug 339127 it is not possible to use special Mozilla css to enable or disable spell checking on a certain form element. There is one problem that I see, what if the page is in a foreign language and expects the spelling to be checked in that dictionary? An idea would be to modify the css style to include an optional parameter which allows you to specify one or more languages which may be used to spell check the field; checked against what dictionaries the user has installed. This would allow the field to be spell checked in only one of the specified languages. There is one problem; in the event that the dictionary is not available, the user would have not be notified or something similar.