But would you find it offensive if I created an elf character and used a bit of the elven language (Sindarin)?
It's not that this would offend me, exactly... it would probably just bother me a little. Mainly because lore-wise, the race-specific languages have been almost lost do to the government's insistence upon all races using common so that they can understand each other. (And so another language, that isn't even one of the races', would be unlikely to even exist.)
This, however, hasn't stopped me from using Klyran or Enkien now and then while playing. I mean, really, it doesn't make much sense that the languages are actually dead by now. There are probably many linguists and the like in Yliakum attempting to reconstruct the languages and restore them to their previous glory.
That being said, I would recommend that you try using one of the languages on the previously-linked-to wiki page (http://planeshift.top-ix.org/pswiki/index.php?title=Category:Language_Projects) for your purposes, as then it would be easier to immerse yourself in -this- world without the worries of offending or annoying any of the other players, presuming they would even care. It would also make it easier for the players of other characters to look up different words so that they and their character could understand what you were saying, and allow them to respond if they saw fit.
Some of the PlaneShift lore is available here (http://www.planeshift.it/History), here (http://www.planeshift.it/Settings), and, well, even here (http://www.planeshift.it/Races), too.
Welcome to PlaneShift, Ariel, and have fun!
By the way, Ariel: I like your sig.