Why did Captain America translate into Captain America?

You got it backwards. Captain America was translated into Captain America. If the name is transliterated according to the ordinary rules, it should be Captain Yamenik. Because of the understanding of the film, the protagonist takes the country name as his nickname, so he translates Yaminik into the well-known United States. Captain itself has the meaning of captain, company commander and captain. The protagonist is in the army and has no rank, so it is more appropriate to translate it into captain.