The reason things are like that is because the vast majority of Internet users speak English and English only. When you speak another language on a website, there usually isn't a way to tell what is being said. Allowing that would be just as good as letting anyone say anything in any language.
edit: As for the traveling the world part, that one has a lot to do with globalization. America is pretty much at the center of globalization, and globalization requires communication. Since Americans aren't going to learn the languages of other countries, as it would be way too difficult to learn something like 30+ languages, they learn our language (English) instead. Of course, America isn't the only one doing this. Obviously England, France, Germany, Italy, and Spain all have their roles.
