You press a button for US, then just tell them that they fucked up and they send it out. Plus it's an american person on the phone so theres no excuse that you can't understand them. Idk if you run a fortune 500 or not, but possibly getting a whole new $90 kit is better than just a flag and heavy bolter.
Here's two examples of how awesome GW is:
1. I got some (5) necron warriors from a friend without the green rods, so I called asking if I could buy/get those, they instead sent me a brand new un-opened box of warriors.
2. I called up because I ordered a unit of shining spears (now just an upgrade kit, but I didn't know) and wondered where the jetbikes were, so they were nice enough to send me a free jetbike squad.
Games Workshop may charge you an arm and a leg for stuff, but if they make a mistake, they are very happy to make up for it.