Wouldn't the more nostr thing to do be a nip and a way that translations get stored on relays?

Clients that offer translations, do so as a distinguishing factor and pay for those translations, not necessarily so that you offer these translations in your client for free but we could solve this on a protocol level.

How about a nip that offers translations for either event IDs or for content hashes?

To avoid second rounds of queries, relays could offer these translations with the translated events, too.