Fed. R. Evid. 604 states that "(a)n interpreter is subject to the provisions of these rules relating to qualification as an expert . . . ." Even though this rule explicitly applies only to interpreters (who translate spoken language), as opposed to translators (who translate written language), no less an authority than Prof. Edward Imwinkelried says, without qualification, that translators are expert witnesses (The Taxonomy of Testimony Post-Kumho: Refocusing on the Bottomlines of Reliability and Necessity, 30 Cumb. L. Rev. 185, 211, fn. 150 (1999-2000)), and the courts generally treat them as such.
Finally, one expert suggests that the court might consult a neutral language expert (such as a court interpreter), on the theory that such an expert will be less likely to be biased (even subconsciously) than a translator retained by a party. (Clifford S. Fishman, Recordings, Transcripts and Translations as Evidence, 81 Wash. L. Rev. 473, 512 (Aug. 2006)).
However, the author also cautions that the court shouldn't automatically accept a neutral court-appointed witness's conclusions as determinative, since the accuracy of that expert's translation is subject to the same standards as those advanced by the parties' experts.