Ich spreche sehr klienes Deutsch, aber es ist nicht so gut.(I apologize to anyone who speaks German, if I butchered that sentence.) I would like to learn Japanese or continue trying to learn German fluently (unlikely either will happen though).
Yes and No. I think that having the same language is great since it makes it a whole lot easier to communicate, but people have trouble getting along even when they have the same language. Just look at all of the civil wars that have gone on throughout the centuries.
Nope, As long as there are people on Earth, there will always be bickering and fighting.
Slang if used correctly is ok, but not when people are to lazy to try and use whole sentences. As for bad language, I personally don't use it and cannot stand when others do. It does diminish what they are saying and typically makes me want to tune them out, since there is no reason for it.