The Name Didacus
Diego was translated into Latin as "Didacus" ("learned person"), which is retrospectively traced to the Greek Διδάχος, Didákhos, "teacher", from διδάσκειν, didáskein, "to teach". Thus, for example, the usual English language name for San Diego de Alcalá (after whom San Diego, California is named) is "Didacus of Alcalá".
This form, and its Spanish equivalent "Didaco", were most likely created in retrospect (that is, to translate Diego into Latin, as opposed to being the source of the name Diego). There are no mentions of Spanish people named Didacus during the Middle Ages. During those times, it was common practice to Latinize existing names, as in Ludovicus for Ludwig (Luis in Spanish).
Even so, some have insisted on deriving Diego from Didacus; nineteenth-century Spanish author Benito Pérez Galdós has a passage in his National Episodes Series 4 (Narváez) that reads:
Su nombre es Didaco o Yago, aunque vulgarmente lo llaman Diego. (His name is Didaco or Yago, but he is commonly called Diego.)
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