Seems all discussions deteriorate into grammar/vocab nitpickery. I like it
I think nesak is the best word here because shilat is for knowing people specifically.
I'm quite sure Ingsve remembers right and David has
specifically said that knowing a language is expressed with
shilat. Might be worth corroborating. I would have thought this has been discussed more than once, but if Ingsve remembers only just and Hrakkar not at all, it's probably been discussed only on some random IRC meet and well over a year ago. The translation on the vocab/dictionary would probably be better changed from "to know (a person)" to "to be familiar with".
I think Finns use neither information knowing (nesat (Finnish: tietää)) nor familiarity (shilat (Finnish: tuntea)) but know-how (devolat? (Finnish: osata)) when speaking about knowing a language. Nevertheless, if the general attitude is similar, you "nesi" the rivers of Germany when you can name then on the map; you "shili" the rivers of Germany, when you have kayaked them up and down a couple of times. You "nesi" a language when you can give a nice overview of it's phonotactics, word order etc.; you "shili" a language when you can effortlessly express yourself with it.
...so at least for me "nesat" would probably be exactly the right word. I'm not really even trying to get to "shilat" type of relationship.