Both, your own attempt and the alleged proposal of DeepL are wrong. (Btw: It's hard for me to believe, that DeepL really produced this output, especially when you say "not sure about the comma." How can you be unsure, when you just need to copy and paste DeepL's output?)
When I enter "That would be the wise thing to do." into DeepL, I get these translations. (The first sentence is the main translation, i.e. the version of which DeepL thinks it would be the best, the others are the alternatives, so it's what DeepL believes to be acceptable.)
Das wäre eine kluge Entscheidung.
Das wäre sehr klug.
Das wäre sehr weise.
Das wäre klug.
They are all correct, and the first one is the one I would have come up with, but they are all a bit what you would call "free". They don't exactly match the English sentence. "The wise thing" is in German »das weise Ding« or »die kluge Sache«, but the words »Ding« or »Sache« are not in any of the translations shown above. And there is a reason for this.
Let's try to create a translation from scratch.
Let's fist analyze the grammar of the English sentence. Here it is again:
That would be the wise thing to do.
- Subject (demonstrative pronoun) = "that"
- Predicate = "would be the wise thing to do"
- auxiliary verb = "would be"
- modal verb = "would"
- main verb = "be"
- predicate noun phrase = "the wise thing to do"
- noun phrase = "the wise thing"
- article (determiner) = "the"
- adjective (attribute) = "wise"
- noun = "thing"
- infinite verb phrase = "to do"
- infinitive marker = "to"
- verb = "do"
And now let's find German words that could be used as word-by-word translation:
that = das
would = würde
be = sein
the = das/die
wise = weise/kluge
thing = Ding/Sache
to = zu
do = tun
If you now glue the German words together in the same order, you get this:
Das würde sein das weise Ding zu tun.
Das würde sein die kluge Sache zu tun.
But now you translated only the words. You let the grammar completely untranslated. So now you have German words in an English grammar. This works surprisingly often, but only because German and English are so closely related to each other. But here it doesn't work.
When you want to create a correct German sentence, you need to find a grammatical structure, that exists in German and that is able to transport a meaning, that is as close to the meaning of the English sentence, as possible.
A first attempt would be this sentence:
Das würde die kluge Sache sein, die zu tun ist.
Here we have:
- That would be = Das würde ... sein
- the wise thing = die kluge Sache
- to do = die zu tun ist
The noun phrase "the wise thing" = "die kluge Sache" is easy, because this works in both languages with the same mechanism.
Also "that would be" = "das würde sein" is strait forward, but in English the modal verb "would" and the infinite verb "be" always stand together, while in German they try to be as far away from each other, as possible. So, in German "die kluge Sache" has to stand between "würde" and "sein".
German has no infinite verb phrases as used in this sentences. There are constructions with infinitive verbs in German, but not this specific kind of infinite verb phrase. So, the English infinite verb phrase to do has to be replaced by something else, and a good first choice is the relative clause die zu tun ist. Both, "to do" and "die zu tun ist" modify the noun phrase "the wise thing" = "die weise Sache". But since we now have a relative clause, we also need a comma to separate it from the main clause.
Here again is our first attempt:
Das würde die kluge Sache sein, die zu tun ist.
This already is a correct German sentence, but no German native speaker would say it. It is clumsy and twisted.
An improvement:
- Das würde ... sein --> Das wäre
The meanings are almost identical. Both constructions use Konjunktiv II to indicate a hypothetical possibility. But »das wäre« has only one verb, so there is no need for a Verbklammer ("verb backet") that encloses other parts of speech:
Das wäre die kluge Sache, die zu tun ist.
The next improvement turns the definite article into an indefinite article, so we no longer talk about the specific thing, but about a thing (one of many possible). This is a good idea, becasue we talk about hypothetic possibilities, and there is not just this one.
Das wäre eine kluge Sache, die zu tun ist.
But when you have to choose one of many things, than you make a decision (decision = Entscheidung):
Das wäre eine kluge Entscheidung, die zu tun ist.
But in German you don't "do" a decision (that's why this sentence is wrong), but you "meet" a decision:
Das wäre eine kluge Entscheidung, die zu treffen ist.
But when you talk about decisions, it is clear, that you have to make them, so there is no need to say it. And then we end with this sentence:
Das wäre eine kluge Entscheidung.
