Obsolete since Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
Selection.selectionLanguageChange() method is a Gecko/Firefox internal method that was exposed to web pages until Firefox 29. It was removed in bug 949445 and should not be used.