[[Localisation guidelines|Don't customise]]
I understand this usage of italics for linguistic purposes (it's the same in many languages), I'm just saying that if a term needs to be quoted as is in all translations then we should alter the English message so that the markup works as needed.
As for loanwords, there is rarely universal agreement so I trust you'll be careful. For Wikibase translations, have you checked wikidata:Wikidata:Glossary/pt? I see the term was translated there: .