One would think that whatever was Columbus' native language would be a clue to his nationality. Unfortunately nobody really knows what his native tongue actually was.
Genoese documents have been found about a weaver named Colombo, however documents in his own hand show that Columbus wrote almost exclusively in Spanish, and that he used Spanish, with Portuguese or Catalan phonetics, even when writing personal notes to himself, to his brother, Italian friends, and to the Bank of Genoa. His two brothers also supposed woolweavers from Genoa never wrote Italian either but Spanish.
There is a small handwritten Genoese gloss in an Italian edition of Pliny's Natural History that he read in his second voyage to America. However, it displays both Spanish and Portuguese influences. Genoese Italian was not a written language in the 15th century.
There is also a note in non-Genoese Italian in his own Book of Prophecies exhibiting, according to historian August Kling, "characteristics of northern Italian humanism in its calligraphy, syntax, and spelling."
In his letters he refers to himself frequently as a "foreigner." The first evidence of his use of Spanish is from the 1480s. Menendez Pidal and many others detect a lot of Portuguese in his Spanish, where he mixes, for example, falar and hablar. But Menendez Pidal does not accept the hypothesis of a Galician origin for Columbus by noting that where Portuguese and Galician diverged, Columbus always used the Portuguese form.
Columbus excelled in his knowledge of Latin, the language of scholarship. He kept his journal in Latin, and a "secret" journal in Greek.
According to historian Charles Merrill, analysis of his handwriting indicates that it is typical of someone who was a native Catalan, and Columbus' phonetic mistakes in Spanish are "most likely" those of a Catalan.
"Chios theory" of Columbus' origin is that he was the son of a Genoese noble family in Greece — which would account for his knowledge of the Greek language — who migrated at an early age to Castillo & Leon near a large Portuguese city, where he adopted Latin, Portuguese, and Spanish (Castellano). As such, this theory explains how he was an accomplished linguist.
Christopher Columbus - discoverer of America