Metadata not being downloaded and also not recognizing local subtitle files

The affected media are already ‘matched’ in the database with that nonsensical ‘language’ selection attached to them.
You must perform the Plex Dance to get some meaningful logs.

[disregard the above]
You re-created your libraries, so the Plex Dance is unnecessary.

Instead, do the following:
Go to

  • Settings - Server - Agents - Shows - TheTVDB

In there, grab the line ‘Local Media Assets’ with your mouse and drag it downwards, so it ends up being at the bottom of the stack of active agents.
Repeat the same under

  • Settings - Server - Agents - Shows - TheMovieDatabase
  • Settings - Server - Agents - Movies - Plex Movie
  • Settings - Server - Agents - Movies - TheMovieDatabase

Then Refresh Metadata on one of the movies.