Great question — answering as the UphillTech team:
Ok, in English this is gonna be weird to explain, but as this link explains, it has to do with the official patent designation. In Portugal, there is the woman "Primeiro-Ministro" (Prime-Minister in Male form), and a woman "Presidente" (President). What is officially registered for the post is "Capitão" (Captain), and as Carol Denvers is a former air force pilot, that's what she is called. There is no "Capitã" for any post in the air force.
Full answer by Gustavo Gabriel: https://movies.stackexchange.com/a/98330 (CC BY-SA 4.0).
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