Well, my main idea was to translate everything: it’s a translation. Names are, of course, an exception, and it is certainly reasonable to leave all names untranslated.
Still, there is a tendency in Pali to use “nicknames” or descriptive epithets that blur the distinction between a name and a description. An obvious example is the surly Akkosaka Bhāradvāja, and various other members of his clan, where the “name” is obviously nothing more than a way of distinguishing that specific clan member according to the personality trait they display in the sutta.
Thus in cases where the meaning of the words seems to have an impact on the subject or manner of the discourse, I have in some cases opted to translate. This also includes cases where, I believe, incorrect translations or explanations have prevailed.
A good example of the latter is the Andhavana. Explained in the commentaries as “Blind Man’s Grove”, and sometimes translated as such, it surely must mean rather “Dark Forest” (or for Tolkien geeks like myself, “Mirkwood”.) Now, a “dark forest” is a gloomy and suspicious place, a retreat into the wilds, and a place where, as any student of mythology would know, one is likely to encounter the demons of the subconscious. It is therefore little surprise that it is in the Dark Forest that the bhikkhunis encounter Mara and repeatedly best him; or indeed, that our hero Kumarakassapa receives such an enigmatic and symbolic teaching. In such a case, keeping it untranslated not merely loses the meaningful context, it allows the much shallower and mythically-insensitive commentarial story to prevail.
In the case of Kassapakumāra, I think the fact that he was a prince is a meaningful part of the discourse. This is not so apparent here, but it is in DN 23, where he is speaking with someone from a similar level of society. We could leave it untranslated, of course, but then, even if someone looked it up, they could think it meant “young”, which in this case it doesn’t.
Checking my translations, I note that I have in fact translated it inconsistently. Oops! I’ll make it consistent. For now, I’m happy to keep it at “Kassapa the Prince”, unless someone changes my mind.