RevvyIts an user error if language used in post and flag displayed for post are not the same.
English Translation => Since the forum bug. will use spoiler tag instead.
If you press "Submit [English]", then thats taken from user UI language. It is expected for user to write in same language most of the time and therefore is shortcut for the drop-down selection.
Drop-down selection however takes its default value from the post you quote or from first post (whichever is more appropriate).