The Scayt spellchecker currently uses the interface language for the spelling dictionary. If you are using a single interface language to edit nodes of multiple languages this is not efficient. The scayt language should be set to the node language, if it's supported by Scayt.
Attached is a patch which adds this functionality. The patch adds the node language to the body form element and that is used to add the scayt_sLang setting. Scayt seems to require a language_locale code so a rudimentary conversion function is added for this purpose. There is probably a better method but it works.