We do not have any limitations on languages for titles we print and distribute via print-on-demand. However, our partners may have issues ingesting metadata using non-Latin character languages. For those languages, please provide the metadata transliterated. For example, when providing metadata for the Russian language edition of "War and Peace" we'd want the title in the Latin alphabet as "Vojna I Mir" not as "Война и миръ" in Cyrillic.
For languages that read right to left, you will also want to alert us before approving your proofs, so that we can manually intervene and create a jpeg with the correct version of the cover.