I would say in the UK that you generally don't say hello to everyone you meet walking down the street - especially in the cities, it would be crazy! However, if you're out walking your dog and you see someone else walking their dog, then most people would acknowledge each other if they are on a quiet street. If you go into a small shop then generally teh people working there will say hello to you and then you should reply - it's rude not to. One thing I do like is that motorbike riders will often wave to each other, even if they don't know each other :)
In France, or at least where I was, you don't say hello to people on the street but you do say hello when you get on a bus and go into a store or up to the till. If you don't then you are very rude.
Where I am in Spain, if you say hello to someone on the street then they will most likely stop and talk to you for half an hour, even if they don't know you! So if you have the time and want to improve your Spanish then that's a great way to do it :)
From my experience, foreign people will often appreciate itt if you go up to them and start speaking to them in their language. It makes teh world around them seem a little scary. I was often doing this in France - even if it was just to ask tehm if they wanted their photo taken in front of the Eiffel Tower! It's very rare that you meet tourists who aren't happy to speak to you :)
But I have to say that I have had days like M_rk_s where I have been in a really bad mood and sometimes people try to force me to say hello. But I can give really evil looks so they generally back off. If they don't, I'll just start speaking to them in another language and smile as I walk off insulting them. I don't think you should force anyone to say hello if they don't want to.