The characters in Beyond the Twelve Words are not speaking English, though their language is probably about as related to English as English is to Early Proto-Germanic. Vocabulary comes from the major world languages of Earth in what the historians of the Allied Worlds consider to be late antiquity (i.e. the present day in real life) but with many developments after that date.
The official language of the Allied Worlds (and its Space Force) is called Standard Imperial, and which is similar to the language called the same thing during the Imperial era.