Bibtex using the wrong language!

Find plain.bst on your file system, copy the file, rename the file to something else. Open the file, add a comment that the name has changed and the current year and preferrably your name.

Find the edition function and replace it with the following block:

FUNCTION {format.edition}
{ edition empty$
    { "" }
    { output.state mid.sentence =
        { edition "l" change.case$ " Edi\c{c}\~ao" * }
        { edition "t" change.case$ " Edi\c{c}\~ao" * }
      if$
    }
  if$
}

Save your file. Put it in the same folder as your tex document (for now).

Change your document to use the new modified bibliographystyle.

Done.

---

Or use biblatex, it picks up the language used in the doument.

You can use babplain for a babel-aware version of plain part of the babelbib package see texdoc babelbib for more details.

You also need to add

\usepackage{babelbib}

so that it defines all the new fixed texts in an appropriate language.