I was going to point out Incubus (1965) as being the only movie in Esperanto, but IMDb also lists Angoroj (1964) and La Eta Knabino. There are also five movies - including Gattaca (!) - which have English and Esperanto listed as languages, and one which is in Serbo-Croatian and Esperanto.
Esperanto is not the rarest language in their database. There three titles in each of Bable dialect (?), Hopi, Malinka, Navajo, Breton, Chaozhou, Grebo, Ojibwa, Shanghainese, Shoshone, Soussan, and Sanskrit.
There are two entries each for Djerma, Khasi, Lao, Maithili, Maya, Polynesian, Rhaeto-Romanic, Scanian, Soninke, Ayamara, Baka, Cheyenne, Dogri, Fon, Haoussa, Ibo, Ladakhi, Lingala, Occitan, Provençal, Serere, Shona, Sotho, Tzotzil, Haryanvi, Kaado, Karaja, Mandingo, and Valencian.
Finally, there is a single entry for each of Corsican, Hassanya, Kandarian, Karbi, Korowai, Kunwinjku, Middle English, Nyaneka, Ojihimba, Shanxi, Sicilian, Tatar, Bodo, Desia, Faliash, Kodava, Kru, Macro-Jê, Manchu, Mende, Miso, Nisga’a, Pawnee, Tamashek, Teochew, Tigrigna, Washoe, Xhosa, Chhattisgarhi, Gumatj, Khanty, Kikongo, Konkani, Kuna, Kwakiutl, Magahi, Marshallese, Micmac, Moso, Nushi, Parsee, Pular, Sardinian, Tarahumara, Tlingit, and Yapese.
(I apologize for misspelling any of these languages.) Now, I’m sure that for one reason or another, a lot of these don’t “count”, but it seems to me there are a lot of unique movies when it comes to language.
…Klingon even has 12 entries…