Technically English has been adopted but nobody recognizes it (or remembers it). My wife, who is a history teacher (now a principal), tells me that when the Constitution was written that English was adopted as the language of all legal documents for the new country. She says that German and Latin were considered but rejected because German was read and written by such a small percentage of people and Latin was considered a dying language.
While that is not an "official" mandate of what the national language is, if a legal document written in anything but English will be rejected then that stands as a pretty strong argument.
The wife is supposed to provide me with some documentation concerning this. Will provide it to all as soon as I have it.