Federation Standard

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Federation Standard, more commonly referred to as Standard or Standard Tongue, and sometimes as Galacta or Earth Basic or Anglish, is the official language of the United Federation of Planets, used in official documents by the Federation government and used as signage on Federation starships. When the Republic of Bajor joined the Federation in 2376, many signs and directories located throughout the planet were altered to include both Bajoran and Standard directions. (TOS novel: Mission to Horatius, DS9 novel: Bajor: Fragments and Omens, FASA RPG sourcebook The Klingons)

Federation Standard seems to be English or a derivative of English, given the English writings seen on the hulls of Federation starships.