node-plex-api VS pasta

Short answer yes, it has a restful api. See projects like https://github.com/phillipj/node-plex-api

My guess is that for both tracks the actual language code used is just Spanish, and it's just the title that indicates esp versus lat. In that case, since there are multiple tracks with the same language, Plex will pick the first track available. You might have to use something like PASTA or Plex Auto Languages for better control over selected audio/subtitles.

It might be easier to use something like PASTA to bulk-select the specific track you want, or Plex-Auto-Languages, which will give you a more Netflix-like experience where changing the track of one episode will go through all episodes in the season/show and automatically select a track of the same language/name if available.

I use https://www.pastatool.com/ for setting the defaults. You can chose to set which track and subtitle is the default for a show or movie.

Outside of that, you'll have to use something like PASTA or Plex Auto Languages to bulk-update the selected audio/subtitle stream.

Since you have the remux, I'd just keep that, and select the right subtitle/audio stream when playing the file back. The major benefit there is that you don't need to have two copies of the same video stream just for different audio/subtitle tracks. The major downside is that if the audio/subtitles you want aren't the default, you have to manually change it for every episode, so for the best experience I'd combine that with a third-party script like Plex-Auto-Languages (a script that will change the audio/subtitle stream for all episodes in a season/show when one changes) or PASTA (a one-time bulk operation tool).

Not within Plex itself, but you can bulk-update a subtitle track for a season/show with PASTA. Plex-Auto-Languages does something similar by changing tracks for an entire season/show if you change the track for a single episode.