The exercise
A Maria e o João trocaram prendas entre si
is translated as
Mary and John exchanged gifts with each other
I was curious how DeepL would translate this and it offered this version:
Mary and John exchanged gifts
I had to tweak the original version to include “mesmo”:
A Maria e o João trocaram prendas entre si mesmo
for DeepL to use “each other” in the translation. This is the version I’d originally come up with myself. Is mesmo actually needed here or are both valid translations?
@pwsteele, “mesmo” is not appropriate here. We only add “mesmo” when the action is reflexive (applied to oneself). In this case, in practice, the action is not truly reflexive, but reciprocal, from Maria to João and from João to Maria. That is what “entre si” (without “mesmo”) conveys in this context 
So DeepL just didn’t do a good job?
I wouldn’t say that, because if I understood well, it wasn’t DeepL proposing the addition of “mesmo”. DeepL’s originally proposed PT to EN translation seemed fine, and I also tested it from EN to PT with no issue. It proposes “entre si” (without “mesmo”) among the possible alternatives The proposed translation to the prompted “entre si mesmos” is also effective in capturing the right idea that the sentence should convey, despite us avoiding “mesmos” in this context in European Portuguese.
My confusion started when I plugged in
A Maria e o João trocaram prendas entre si
In DeepL and got
Maria and João exchanged gifts
instead of
Maria and João exchanged gifts with each other
I didn’t notice DeepL also suggested this version in their alternatives list…
@pwsteele I would say that Deepl is right, in that “exchanged gifts” is the more idiomatic English and that “with each other” is redundant, unless there was any possibility of confusion caused by other parts of the text. English is very fond of the missing, but implied, phrase, clause or word.
Yes, “exchanged gifts” is enough to express the meaning of the sentence. The main purpose of adding “with each other” in the exercise is having better alignment with the Portuguese sentence.