The metadata language for newly added items is defined in the properties of the library. “Edit” it, to see the language selector.
It’s not possible to change this en masse for existing items.
The language is part of the “matching” to a metadata source. So to change that you needed to do either:
- perform the “Fix Match” procedure on each item separately
- Plex Dance the affected items
- Remove the whole library from Plex and recreate it, this time with the right language setting
