For a language, make it matter now
I'll also advocate the "just tell them" approach, if only because it is very, very hard to make sure that clues and hints are as clear to the players as they are to you, the DM. And it's not like the giants are a huge secret in Storm King's Thunder. But I totally get not wanting to be so blatant, so I have one other option for threading the needle:
Give your players some reason to take Giantish right now. It's easier than trying to get them to keep the language slot open (and easier to explain in-game), and doesn't require you to nudge them towards something they won't understand until the future.
I would do this with a side quest or mystery, but I'm sure that there are other options. A mysterious note, written in a language none of the PCs know, has a good chance of prompting them to figure out its language and what it says. A side quest that involves lots of Giantish communication (in speech or in writing) can be a great motivator, especially if you hint that there is a desirable reward for the quest's completion.
In a pinch, you can help players tweak backgrounds in such a way that they make Giantish an obvious choice, but that might be too close to just telling them for your taste.
Extra languages are easy to forget about, and they often don't lead to obvious rewards anyways. If any of your PCs have extra language slots, for any reason, it might not take much to persuade them to take Giantish.
For Favored Enemy, maybe don't worry about it so much right now
Rangers get multiple enemy types for Favored Enemy, and if you're not transitioning to Storm King's Thunder until level 5 anyways, your ranger can just take giants as a Favored Enemy at level 6.
This can be a tidy solution if giants are rare in your current campaign-- taking them for Favored Enemy is kind of a wasteful choice until they start showing up. This also automatically grants Giantish as a language, so that can also solve the language problem pretty early in SKT. This, of course, is pretty specific to D&D 5e, and so isn't great general advice.