Update:
I managed to fix the “0 items in the shopping bag” problem by myself
But the following strings still doesn’t work
1) Size attribute – (The word “Size” in product page)
2) “Category:” – (In Product Page)
2b) “Tag:” – (In Product Page)
3) “Returns & Exchange” – (In My account page)
4) “Edit address” – (In My account > Address Book page)
Again, I am using WPML’s string translation. I saw some user had an exact same problem as mine in the other thread:
http://support.swiftideas.net/forums/topic/some-strings-wont-translate-wpml/
Is it better to use PO / MO method to translate? Will I need to start all over again if I use PO / MO? Please advice. Thanks!