How can I make VLC auto-select subtitles in an MKV file?

For me, it worked with the following settings:

Subtitle track = 1

Subtitle Language = English

Subtitle track ID = 2

Hope you'll find this useful. (If it doesn't work, click on Tools, Media Information, Codec and look which stream type is Subtitle... That is your subtitle track ID! )

The number you are inputting in Subtitle Track ID within the mkv container. Usually Track ID 0 will be the video stream and ID 1 will be the audio stream. Thus, starting from 2, you might have the embedded subtitle streams. If you have multiple audio tracks, however, those will be pushed back even further. Specifying the number directly like this is thus rarely a good way to do it, as it can vary from file to file unless you specifically encoded them all to be uniform.

I was unable to find out what exactly the "Subtitle track" setting does, but I would advise to leave it at -1, the default and revert the "Subtitle track ID" setting to -1 again as well.

If you want to automatically select English subtitles, then put "English" (not "en") in the "Subtitle language" field and it should automatically select the English language subtitles from your file if it is available. This likely does not work if the language is not properly tagged in the file, and I don't think it is possible to select the second English subtitles as you specified automatically.