Weird. For me only subs in the FIRST preferred language are auto-checked! (Pro 2.57.1044beta - 201010011004)
That's exactly how I would expect it to work!
So why it acts differently for you
P.S.:
Adding languages is no problem, but it so seems that in order to change the first default/preferred language, one indeed has (as mentioned by
adicoto) to first empty the list and then re-build it. (Otherwise the old/previous language will simply re-appear.) This seems an unintended feature or - in other words - simply a bug
to me!
(Good that a work-around is already available!)