Translation:Let's swim together again next month.
Maybe I'm not the swiftest learner, but whenever Duolingo tells me "will we go to [place] next month" is not correct, but "shall we go to [place] next month" is, I feel really nitpicked at. In casual English, hardly anyone says "Shall we?" unless you are being pompous on purpose or trying to impress a date or whatever.
Does "go swim" sound weird to anyone else? Like, I guess it's fine grammatically and semantically, but activities are usually expressed in a progressive tense when coupled with go/gone/went. "Let's go shopping." "I have gone hunting." "I went bowling." While you can substitute plain for progressive in present and future tenses, you can't use plain for activities in perfect nor past tenses. I.e. you can't say "I have gone hunt" or "I went bowl". If you wanted to use plain tenses in those instances, you'd say "I have hunted" and "I bowled". Likewise, I would either say "let's swim" or "let's go swimming".
I was looking for this. Yeah, "let's go swim together again" sounds wrong to me too, but maybe that's my southern dialect talking. "Let's go swimming together again" or "let's swim together again" sound fine to me, but not "let's go swim together again." I was kind of peeved when it told me "Let's swim together again next month" had a typo because I didn't put the word "go."
It ABSOLUTELY is necessary. There is no plural form in japanese, and without "together" would mean that you two perform the activity apart.
For instance "let's train our tennis skills next month" doesn't necessarily mean you do the activity together, it means you train to become better.
There's something weird about Duolingo's error detection algorithm. I input let + 's and the answer flagged it as an error saying I should input let's except that's not even one of the option. I'm honestly considering quitting Duolingo altogether because I felt that not only will I not get better with Duolingo, I suspect some of the usage might be wrong alltogether.
Some of the stuff in Duolingo is a little weird, so if you decide to use it I would suggest double checking on another site if something seems off. The way I started learning Japanese was entirely through Duolingo, but now I only use it for practice and vocabulary. I bought the physical version of Tae Kim's "A Guide to Japanese Grammar" and now I can read moderate-level books and manga with little difficulty (if it has furigana.) The downside is that it's a little difficult to understand unless you know linguistics terminology. I had some trouble with certain explainations. I really only bought it because it was one book and it was cheap, it's even available for free online. If you're willing to spend some money on it, here is a list of good textbooks: teamjapanese.com/best-books-to-learn-japanese