From "Introduction to Spanish Translation" by Jack Child (ISBN 0-8191-8589-2

"In government correspondence official slogans ("lemas") are sometimes used either in the letterhead or more commonly just before the signature block. These usually reflect an event or saying of some historical significance, and should be translated between quotation marks. The English speaking businessperson may find their use a little startling, and the translator may wish to add a brief footnote explaining the significance."

As you know, universities, generally also include their "lemas" in all official correspondence. Whatever translation solution you choose (and the UNAM motto has been the subject of much discussion), I agree with Child's statement "The English speaking businessperson may find their use a little startling,"—perhaps it is truer to say puzzling than startling—and I always follow it in a translation with a note such as: [This is the historical University of Mexico motto, which appears on all official university correspondence.]

http://www.proz.com/kudoz/spanish_to_english/law_patents/811...
Note that the asker decided not to translate the motto.