If you want to take a photograph of someone, ask first. If they don’t speak the same language as you, make eye contact, show them your camera, and wait for their response. If they nod their head, “yes” take a photo or two. If you think it’s appropriate, you can show them the picture too.
Many people, especially in impoverished parts of the world, don’t have many pictures of themselves, so it's a cool experience for them to see it. If someone shakes their head “no,” respect their privacy and move on. There are plenty of people who wouldn't mind being photographed.