Are there any guidelines/best practices concerning the insertion of paragraph breaks in long paragraphs (I don't know the reason, but the usage of very long paragraphs in German texts seems common at least in texts from up to the beginning of the 20th Century) when translating from German?
Very long paragraphs are a consequence of very long sentences. Such a very long sentence might subsume all points of a idea and as such be a whole paragraph. This is mostly bad style. It'd be better to take that sentence apart, but the author didn't wish it that way. But he might have realized, that it seems rather strange to have a paragraph consist of only one sentence. So he decides to add one or two other (such very long) sentences, which makes the paragraph very long.
If you add extra paragraph breaks, you are not only translating but changing the original text in a way, the author might not have wanted. Do so, if you think it reasonable and don't fear the author's wrath. Leave it otherwise.
I remember very long paragraphs (20 pages and longer) in books from Franz Kafka (»Der Process«, »Die Verwandlung«) and Immanuel Kant (»Critik der reinen Vernunft«). And I am sure that many more authors wrote long paragraphs too.
If you want to translate such old texts you should not break those long paragraphs. They are part of the style that those authors used intentionally. Breaking the paragraphs means changing the style.
But maybe you want to create an easy-reading version of those old texts. This can be a good idea to bring those texts closer to young readers. In this case you also have to break the long sentences (some of them more then 5 pages long) into shorter ones, which means that you have to re-write the complete text.
If you do so, the first sentence in a new paragraph should express a new idea, a new statement or a new point of view. The rest of the paragraph should describe this new element. This rules was valid in times of Kafka and Kant too, but authors in those times had another imagination of what was a new statement and what was its description.