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.