The master Synology FAQ is here:
The one for creating the Docker overlay on top of shared folder PlexMediaServer is this:
How to use MKVtoolnix to filter your media
There are multiple tutorials on the net for how to do this for your machine.
In a Nutshell, We find the track(s) we don’t want and UNCHECK the box for that track.
When we click “Multiplex”, MKVtoolnix will read the file; copying only the tracks we want to the output file.
When it’s done, we give that Output File to Plex. All the other junk in there will be gone 
Here, you can see (for example only) I unchecked the SRT subtitles. You will want to uncheck the boxes on ANY tracks you don’t want.
It’ll make it easier to stream plus you will be saving space.
