Error while set language, despite package is already installed

I’m writing a document in R markdown and using latex for figures and tables. I need the captions in Portuguese, so I try this:

R markdown file:

---
output:
  pdf_document:
      includes:
            in_header: mystyles.sty
  html_document: rmarkdown::github_document
  word_document: default
---

mystyles.sty file:

usepackage[portuguese]{babel}

However, when I build the document using rmarkdown::render("filename.Rmd") I got the error:

! Package babel Error: Unknown option `portuguese'. Either you misspelled it (babel) or the language definition file portuguese.ldf was not found.

As I’m in Ubuntu, I ran:

sudo apt-get install texlive-lang-portuguese

which returns that I already has this package installed:

texlive-lang-portuguese is already the newest version (2015.20160223-1).

I also tried to run sudo apt-get install texlive-lang-european which returns the same

What am I doing wrong here?