Q: Why is The Urantia Book translated into other languages when its authors favor a one world language?
A: The very first paragraph of the book states that the authors were "authorized to translate into the English language..." I think one reason that the authors chose English is that it is a very fluid language, ever adaptable and full of derivative words from many languages.
This is surely no accidental or unthinking choice. All we have to do is look around us and we see that many countries educate their children in English, and English, I have heard, is becoming the language of business as well. And we know how important commerce is to establishing global unity and cooperation.
Jesus was conversant in several languages, and this is a trend that is developing throughout the world. For example, here in America, English co-exists with Spanish in many communities. Language is an evolutionary development, and it is going to take a long time before we reach the stage of a "one tongue" world, which is a feature of the ages of Light and Life.
Some may question the decisions that have been made regarding translations into other languages, but it seems to me that if you want to reach a wide audience, you do what is necessary. Jesus said that we were to spread this message to all the world... better that people should get this message in their own language right now; later on, they may be able to master English because of it. But regardless, they will have gotten the good news.
We are given no concrete guidance in the book regarding this issue, but since the Urantia Revelation is meant to feed humanity for the next millenium or so, I think translations in other languages at this early stage is entirely appropriate. In the end, the message of The Urantia Book—in any language—must be translated by each individual into "the language of the spirit..." that Jesus spoke of.
Thanks for this interesting question!