I've had the same problem, not so much so with the English, but more so with the call centers. I too was told that a flight a couple of weeks out was "under airport control." When I called back the next day and spoke to a U.S. agent she just laughed and changed my seat assignment in about 30 seconds. My experience suggests the difficulty is that United's overseas agents have very little access to information or flexibility in what they can do. Contrast this with Amazon's call centers where the overseas agents are empowered to do their jobs and the difference is night and day.