It's any language learned after the first language or mother tongue.
Are you sure? For any language acquired after my mother tongue, I know the expression "second language" (not "secondary").
I would have thought that this might be in a computer context, a secondary (programming) language or something like that.
But in order to help, dear Gaia Seca, the forum community will need some context. Where did you find this expression, what kind of text is around it? Many thanks, muchas gracias (unfortunately, I don't speak Portuguese at all; came here out of sheer curiosity )
Source: WikiAnswers.Question: Primary language is spoken while secondary language is written?
Answer: Your primary language is the one most often spoken. Schools dealing with ESL populations and immigrant parents regularly query what is the primary language spoken in the home to help with home school communications. A secondary language would be any other language you spoke, or if more than two languages are involved the one most next often spoken after the primary language.