It may well be that responding in Brazilian Portuguese or via a Brazilian lawsuit would have weakened their case that they do not have a presence in the country

It may well be that responding in Brazilian Portuguese or via a Brazilian lawsuit would have weakened their case that they do not have a presence in the country. And yes, it's also possible that it's just an arrogant law firm (always a possibility), but it's also possible that their counsel advised them specifically not to retain a Brazilian-speaking attorney, or sworn translator into Brazilian Portuguese (who typically would have some ties to the country). Another possibility, frankly, after the last time ended up with one of their executives arrested for contempt of court, is that Facebook/WhatsApp may not have wanted to risk the freedom of some poor freelance translator in Brazil, who risked being hauled off to jail for having his name on the translation (required for legal translations in most countries).