Sorry, I don't understand why Translate would have anything to do with licenses.

Well, the content of e.g.
https://meta.wikimedia.org/wiki/MassMessage
is under CC-BY-SA, which requires attribution when someone reuses the a content licensed under that license. This applies to translations, but the extension doesn't allow us to give credit to the authors in the edit summary, so it should do that itself.

Luis: License related question - any recommendation for this by Legal?
(Feel free to unCC yourself again if this is out of scope.)

Where the original work is substantial enough to be copyrightable (which is certainly the case for a big thing like this example), the translation needs to comply with the license of the original work.

I'm not clear how best to do that here; there are a lot of different ways you could link back to the original history.

All the authors can be found in the history of the relevant pages. The relevant pages are interlinked. Is that not enough?

I'm not familiar with these particular tools, but at least in a quick glance, I found no links from the translated page to the original page - you had to know how to manipulate the URL to get to the original material. I may be missing something, though?

Not a (Translate) bug.

1. The edit summary is technically available for you to customise, Translate doesn't alter it.
2. The header of the page itself says it's a translation from the source page. https://meta.wikimedia.org/wiki/MassMessage/pt-br This link is more visible than any link in action=history and therefore satisfies both cc-by-sa and [[m:terms of use#7c]].

https://meta.wikimedia.org/wiki/MassMessage has the language links near the top of the content. You can see the translation activity of a translation page at https://meta.wikimedia.org/w/index.php?title=MassMessage/pt-br&action=history .

Internally, the Translate extension also stores the translations in units in pages like https://meta.wikimedia.org/wiki/Translations:MassMessage/8/pt-br . These translation unit pages are usually not accessed directly. Translators modify them via a view like this: https://meta.wikimedia.org/w/index.php?title=Special:Translate&group=page-MassMessage&language=pt-br&filter=&action=page

(In reply to Nemo from comment #7)

Not a (Translate) bug.

1. The edit summary is technically available for you to customise, Translate

doesn't alter it.

So, where can I alter the summary using the translate extension?
https://meta.wikimedia.org/w/index.php?title=Special:Translate&group=page-MassMessage&language=pt-br&filter=&action=page
So far I was not able to find a way to edit the summary, as can be checked on e.g.:
https://meta.wikimedia.org/w/index.php?title=MassMessage/pt-br&action=history

(In reply to Helder from comment #9)

So, where can I alter the summary using the translate extension?

Parameters action=edit or tux=0, or API. Cf. bug 46924.