The Pearl of Speech is a 600 gp slotted item that lets you speak and understand a single, specific language. It also produces a Command spell (DC 11) 1/day. Since the tome is slotless, it should cost no more than twice what the pearl would without the Command spell, maybe a little more since you can use multiple tomes to learn multiple languages.
One of WotC's web adventures (
A Dark and Stormy Knight, download no longer available, unfortunately) had a 400 gp magic item, the Translator's Ring, that gave understanding and literacy of two languages for 400 gp.
So I'd say 600 gp at the absolute maximum, probably less. It's already straining the limit of what PCs are likely willing to pay for it in a standard campaign, even one dealing with a language barrier, what with low-mid level magic like Comprehend Languages and Tongues that can temporarily bypass it if it's an issue, anyways. There are very few character builds that cannot spare 1-2 skill points to learn an important language if it keeps coming up.