I work for the Australian Defence Force School of Languages. I help arm Australian soldiers with communication skills, rather than destructive weapons. I have yet to learn a language (I'm a test coordinator) but I can say the following with confidence. Most disputes come from a communication breakdown. If you open the communication lines, the results you get from people are astounding.
Think about it this way. The only way to interact with people, is by communicating, be that via speech, body gestures etc....
If you improve your communication skills, you are able to communicate and receive communication more directly, lessening the chance of misinterpretation.
Simple example is Kevin Rudd's address to the Chinese President. While the greeting could have been spoken in English, he showed he was willing to make the effort to accept part of the Chinese culture within himself, and that earns respect. Best part is he would have had the Chinese president interested in him because there's a chance Kevin Rudd has a good understanding Chinese culture.
It's a strange assumption that English should be learnt by the other party, or if they are already fluent in our language, we are not required to learn their language.
It is the same with bicycles. Watch someone new to bikes in a store while a salesperson speaks about groupsets and derailers and different types of spokes - the terminology goes past them and they become lost. They don't respect each other because the poor newcomer thinks the salesperson is being arrogant or trying to pull the wool over his/her eyes, and the salesperson thinks the newcomer is a too much effort or a muppet for not understanding. (Obviously it can be more subtle than that). Learn the terms and you can talk at length about it all. May even become best friends. Who knows.
If you want to test what sort of difference tone makes, call a dog to come to you in your nicest, gayest, ultra happy voice and watch the reaction. You'll get excitement. Now say the same words in the loudest, angriest voice you can muster. There's a good change the tail will go between the legs and the dog will not come. Humans are no different. If I screamed and swore at you to ask the time, you'd probably want to avoid me. If I was soft and polite, you wouldn't have a problem.
How much fun is it to talk to a drunk who can barely string a sentence together?
Anyway, I've rambled on too much.