You’re still unable to set the original_title field through the API.
This has been an issue for almost 10 years now.
This has also been reported several times:
Bug Report: Unable to update original title in TV show and season summary - Third Party / Development - Plex Forum
How to set original title for tv show programmatically? - Third Party / Development - Plex Forum
Could we please have an update to this? Especially when handling a big library of anime, deciding if you want to have them all in English or all in Romaji, or having to manually add the Romaji as Original Title to over 400 entires… It would be much better if the agents could be allowed to write that data (which seems possible on Movie libraries, but not Series).
Best regards