In The Hobbit: An Unexpected Journey, the Dwarves present Bilbo a contract. It's presumably in Westron, but through the magic of Translation Convention, it appears on-screen in its entirety in English, which has allowed a lawyer to analyze it. It turns out to be a fairly solid contract, except for one thing:

Disputes arising between the Contract Parties shall be heard and judged by an arbitrator of the Company’s choosing and all pleas shall be pleaded, shrewed [sic], defended, answered, debated and judged in the Dwarvish Tongue

The "Dwarven tongue" is Khuzdul, an "Afro-Aulësiatic" language not provably related to more widely known languages of Middle-earth. It's at least as distant from Westron as Hebrew is from English (notwithstanding the false cognate quackery that is Edenics). Only a handful of non-Dwarves ever learn Khuzdul, as Dwarves use Westron in public. As Royal Canadian Bandit pointed out in a comment:

there are no non-dwarves who speak any Dwarvish, aside from a handful of words. In the LOTR books, Gandalf refers to it as "the secret dwarf-tongue they teach to none" while trying to work out the password to Moria. So poor Bilbo would get a hearing where his advocate, his opponent, and the judge were all dwarves speaking a language he couldn't understand.

Why would Bilbo in his right mind agree to arbitration in Khuzdul when the Dwarves are fluent in Westron? True, Westron's ancestor Adûnaic has some Khuzdul loans, but I can't see how Bilbo could follow the proceedings from just that. Or is there some difference in knowledge of languages between book-verse and movie-verse?