'
Japanese and Koreans are the best in common courtesy. Keep on apologising for troubling you and keep on thanking you for your help, even just attempt to help that ended up no help at all. Indians? If you say you don't know to their questions, they'd just turn away as if you've never existed. That's why I'd stare back in silence to whatever help or question they ask. Same result. They'd just walk away.
I agree that the Japanese and Koreans are the most courteous group of people. As for the Indians, that is why I give them the shortest, vaguest answer I can think of when they ask. Note, not all Indians are like that, the colleagues and the Ah Neh mei mei at my house are very civilized. I suppose it depends on your luck. Once an Ah neh came up to me at the Shell station asking me which fuel is better, V-power or Regular. I looked at his lemon Toyota and just said, "It depends on your car" and continued pumping fuel into my car. He stood there for a while, can't think of anything to say and went to another fellow asking the same question. I don't think they turn away because they are rude, they are just not sure what to ask next. They Ah Nehs in general are very talkative in nature. If you give a too precise or concise answer they would have follow-up question. The best policy in dealing with them I find is to give the shortest and vaguest answer you can think of, leave them stumped and they won't bother you again. If you adopt the policy like the Japanese/Korean of being overtly courtesy you might end up spending 1-2 hours answering their incessant questions on a simple subject.