You can get the official list of supported languages from this endpoint:
https://developers.themoviedb.org/3/configuration/get-primary-translations
As of today the endpoint returns the following ISO codes:
"ar-AE",
"ar-SA",
"bg-BG",
"bn-BD",
"ca-ES",
"ch-GU",
"cs-CZ",
"da-DK",
"de-DE",
"el-GR",
"en-US",
"eo-EO",
"es-ES",
"es-MX",
"eu-ES",
"fa-IR",
"fi-FI",
"fr-CA",
"fr-FR",
"he-IL",
"hi-IN",
"hu-HU",
"id-ID",
"it-IT",
"ja-JP",
"ka-GE",
"kn-IN",
"ko-KR",
"lt-LT",
"ml-IN",
"nb-NO",
"nl-NL",
"no-NO",
"pl-PL",
"pt-BR",
"pt-PT",
"ro-RO",
"ru-RU",
"sk-SK",
"sl-SI",
"sr-RS",
"sv-SE",
"ta-IN",
"te-IN",
"th-TH",
"tr-TR",
"uk-UA",
"vi-VN",
"zh-CN",
"zh-TW"
I tested some of the languages with the genres endpoint:
https://api.themoviedb.org/3/genre/movie/list
Some of them return null
for the genre names (meaning they have not been translated).
You can see an overview of the translated material at https://www.themoviedb.org/contribute.