Can anyone else than drow learn Drow Sign Language? or is it a language that only drow can learn because of, I dunno... dexterity or some other physical conditions?
Yes, RAW others can learn.
Drow Sign Language is a bonus language for drow; others have to spend skill points to learn it. It has no alphabet or written form. (FRCS, p. 13)
As always DMs can rule however they want and may require you to find a willing drow to teach you. (If I allowed it, I would require this.)