I am not sure anymore whether this is a bugreport or not. Anyway, it might be of use to other shop owners.
The cart weight is not converted between LBS and KGS when switching from English to German/Spanish. The weight count remains the same.
How to reproduce: Install Zone Rates shipping module as this one is showing the cart weight during checkout. Put a product into cart and go to checkout. Zone Rates will show the weight. Go back to catalog, switch language and check out again. The weight unit has changed, but not the count.
This cart behaviour might confuse customers when you run a shop with both units in use and a shipping module which shows the cart weight. A shop which uses just one unit or doesn't show the cart weight is not affected at all.
The weight unit text string originates from catalog/includes/languages/CHOOSE_A_LANGUAGE/zones.php. So I assume the Unit Weight setting in Admin Panel is for admin side only.
I stumbled across this behaviour when i was playing around with a very easy to integrate shipping estimator:
Another Shipping Estimator - Cost Shipping in Shopping_cart Frais de port dans le panier