Both languages are considered to have a default. Period.
You cannot have an audio language included that doesn't have a default.

Matroska spec dictates this. Even if you wrongly do not give a default to each audio language, the matroska spec assumes you're an idiot and assumes all audio for that language has default flag even if its set to default: no.

Instead of telling me to go back to school, you should learn how to read what the spec dictates and what it says the default flag does and how it works, because you clearly failed this.

https://www.matroska.org/technical/notes.html <- scroll down to default flag.

"The “default track” flag is a hint for a Matroska Player indicating that a given track SHOULD be eligible to be automatically selected as the default track for a given language. If no tracks in a given language have the default track flag set, then all tracks in that language are eligible for automatic selection. This can be used to indicate that a track provides “regular service” suitable for users with default settings, as opposed to specialized services, such as commentary, hearing-impaired captions, or descriptive audio."

The default flag does not mean "this is the track that plays by default". Again, all audio languages included in the file are forced to have their own default track and if you fail to utilize the flag properly, it gives default to all audio of that language. There is no way around this failsafe.

In the case of this file, both the English and Japanese audio are assumed to have default set to yes even though VARYG improperly set the it to no on the English track.

I never said you can't change your settings in your player for preferences. I simply pointed out the fact that both tracks have default set to yes even though English says no, because the matroska spec dictates that all language options have a track with default so the whole "English should be THE default" argument is wrong, they both are default despite VARYG failing to tag properly.