Two things:
1 - It's a comic book movie based in a Universe that has talking trees, talking racc00n's, Space gods, and Giant green rage monsters. A city remaining hidden from the rest of the world shouldn't be that huge of a stretch.
2 - Wakanda is supposed to be the MOST technologically advanced nation on earth, they have shyt that the rest of the world (including Tony Stark) couldn't fathom. They can hide their capital city because they have tech that it's years ahead of everyone else AND they murk anyone that tries to cross into their nation. Klaw clearly said in the trailer that he was the first person to see the nation and make it out alive.
In the comics at one point an Alien race tried to invade Wakanda, and this is how shyt ended for the invaders: