Actually, a large mountain can be seen in the background during the 'Decepticons in Africa' sequence. I wanna' say it's Kilimanjaro, but I don't know much about chemistry...

But yeah, in the novel Sentinel and Optimus speak in the mountainous landscape of Canada (WOOT!). I'm not really sure where it is they speak in the movie, but it doesn't really matter. As someone already stated, it was simply for Optimus to show Sentinel the natural beauty of Earth. I, personally, didn't confuse it with Africa.