Nehekeran is of course the language of the Tomb Kings (... as far as I am aware, Lore Nerds feel free to correct me), and the language of Necromancy, and as such it's likely that there might be texts in the bookmines of Drakenhof written in that language. It's probably not super urgent to learn this one, but I think it'll be cool. Egrimm probably has a passing familiarity with it, although I doubt he can speak and read it well.
I also want to learn Classical for the purposes of BOOK—it is the oldest know human language in the Old World, and the language of choice for many scholars, priests and wizards, and as such is a language many tomes and grimoires are written in. Mathilde had the option of learning this as an apprentice (which I believe we learned when we first met Cython), but instead studied Old Reikspiel. Classical is also the root language for Tilia and Estalia.
Since we've got this shiny new-ish Polyglot trait and the only thing we've really done with it so far is speak a language we already knew at the Elves, I figured putting aside some time to learn these languages as a personal action would be worthwhile. Especially since each cultures book bonus caps at +5, and we mainly have access to Imperial, Bretonnian, Eonir, Dwarven, and Skaven texts at the moment, and some of those are harder to get hold of than others. Adding more languages will only strengthen us.
The only downside is that I don't know how much AP learning a language through study consumes, even with the Polyglot trait.