Completely irrelevant to my point...apparently I need to explain further.
The fact that Luke has never been off the planet means nothing. He's obviously interested in interstellar events and space travel, as evidenced by the dialogue establishing his character early in the movie. The Kessel Run is not some big secret that nobody knows about - it would be well known and the concept of smuggling runs and sticking it to the Empire would be idolized by many, especially someone like Luke. So it's well within reasonable logic that he would know some details about the Kessel Run, including how far a normal trip would be and how much the faster (more idolized) smugglers could shave off it.
So now on to the term "parsec." Are we to believe the common language in a different galaxy is somehow magically English? Oh wait, if it's watched in Germany, it magically becomes German with humanoids no longer having the same mouth movement to word sync? Wait, are these people even human?? WTH is going on, NONE OF THIS IS POSSIBLE, OMGWTFBBQ.
Movies require a certain amount of disbelief suspension in order to function properly. Part of that includes known terms that are being used in a familiar fashion, even if they would carry a totally different meaning in the movie universe - like parsec. Suspend disbelief.
So we've established that the term is completely valid within framework of disbelief suspension required for just about any movie to function. We've also discovered that the movie establishes Luke's interest in interstellar events and space travel (If you don't believe me or remember, go watch the movie again, it's not hard to pick up on), and would probably know about the Kessel Run and potentially even idolize some of the people sticking it to the Empire in that way (bad Empire, space travel, danger, adventure, doing what you want when you want, not being stuck on a moisture farm, etc...not hard to connect those dots, either).
Any other questions?