And then in later seasons, we could have had Sam and Dean with better understandings of and familiarity with the language patterns of Enochian. They wouldn't be able to understand the words (at least, not ALL the words), but they would at least be able to recognize the vibe of the conversation.

They could have been able to pick up SOME words and phrases over the years, especially Sam, since he had been in Lucifer's cage for so long...and some words and phrases would have been basically impossible to translate into any human language because angelic existence is so vastly different than that of humans.

We could have gotten Cas and Jack speaking with each other in Enochian. Maybe Sam and Dean noticing that the way Jack speaks is quite different from the way that Cas does (like, the cadence is noticeably faster and the intonation seems almost frenetic) and getting concerned because what if it's somehow Lucifer's influence, what if Jack's Enochian is some hell-infused version of the language?

And then Cas explaining to them that actually, Jack's way of speaking Enochian is completely normal because angelic fledglings just have different speech patterns than adult angels do. (and yes, nephilim are considered angelic even though they're also part human)

We could have had Cas regularly use Enochian terms of endearment when speaking with Jack. Maybe Sam and Dean pick up on it—they don't understand the words, but they recognize that Cas uses them a lot with a tone that they've learned is loving. Maybe at some point, they ask Cas what the words mean. Some of the words have human equivalencies, like "little one" and "darling child." Maybe some are just diminutives of Jack's name (or diminutives of Jack's angelic/Enochian name, if we think that maybe he has one).

There are a few terms of endearment that really can't be translated in a meaningful way into English or any other human language. Cas tries, but the words end up feeling clunky and vague. Eventually, Cas just shrugs and tells them "it's more affectionate in Enochian."

Maybe Sam and Dean eventually start to learn and use a few simple Enochian words and phrases when they're with Jack and/or Cas. Things like "good morning," "good night," and "thank you/you're welcome". Maybe they start to learn more about the intricacies of angelic culture and how those intricacies are reflected in the Enochian language.

Their pronunciation isn't perfect, but it's understandable and the effort is very much appreciated. Maybe it makes Cas and Jack feel comfortable enough to sprinkle bits of Enochian into conversations with Sam and Dean. Maybe Cas starts using Enochian endearments when addressing Sam and Dean.