Languages

[Dorganath]

Florane is one, mostly because nobody speaks it.  It's the language of the T'oleflor.  Though I suppose if you find one, you can ask for language instruction.

Shadow (not listed above) is another, because for those few who do speak it, it was given as a plot quest reward, and it is currently unlikely that anyone else, outside of some potential plot-level events, will be able to learn it.

[Timmy0420]

so could a shadow dancer (or whatever class) learn shadow from someone else who speaks it? or maybe arrange a cdq on the shadow plane or something?

[Acacea]

I don't believe so. And with the new planes and all that I doubt there is a "Plane of Shadows," but still, even with equivalent, it would have to be a VERY special case as those who have it learned it under extreme circumstances, and not because they wanted to, but rather because the shadows wanted to be understood. If that makes any sense.

[pejsaboy]

It makes sense if you've heard the story

[Dorganath]

so could a shadow dancer (or whatever class) learn shadow from someone else who speaks it? or maybe arrange a cdq on the shadow plane or something?

Short answer: No.

Long answer: *points up to what they said*

Less obscure answer: Those who do speak it aren't really qualified to teach it...they just...know it.

Adminsitrative answer: Leanthar gave it as a quest reward and that was it's sole intended purpose in the hands of PCs.