English is a mess because it is a mixture of languages from different ethnic groups: Anglo-Saxon, Norse, Celtic, and Norman. It's been used for cross-cultural communication ever since it was identifiable as English.
Hackernews armchair linguists seem to think an ideal language for world communication can somehow be engineered from some small set of primitives, like Scheme -- resulting in suggestions like that we all start speaking Toki Pona -- but the reality is that human communication is messy and the most practical languages tend to be messy ones. Some linguists have observed that trade pidgins develop English-like morphologies, even when English is not one of the contributing languages.
(And even Scheme got messy; see R6RS and R7RS...)
As for Japanese, I love it, it's beautiful, but as even any weeaboo knows, Japanese language is very bound up with Japanese culture. You can't simply ignore or elide the politeness bits; where you stand in society very strongly influences what you say. The fact that English is largely free of this baggage helps make it an effective language that people around the world pick up and use for trade, especially when different ethnic groups are involved.