I was talking with @E_net4 about this question elsewhere on meta. The proposition is:
The site is open to discussing translations from X to PT for any language X (provided the discussion is conducted in EN and/or PT). Translations from PT to EN are also on-topic; and "closest word" questions; but all translations from PT to X =\= EN are off-topic.
The only clause anyone might dispute is that
Translations from PT to EN are on-topic, while those from PT to X =\= EN are off-topic
I'll put my argument against this below so it can be voted on alongside any arguments in favor. I think it's worth clarifying and I won't be offended by downvotes on the position I advocate below.