Default language/track change

[rediscover]

I have an anime series which I watch on VLC because they are in .mkv format. These episodes have two audio tracks, one in english and one in japanese. Unfortunately, I don't speak or understand japanese so I always have to change the track to the english one. Sometimes, the episode's english track isn't always the first track. Some episodes have the english track as the first, while some have it as the second. This is annoying for my main problem, which is that I want to convert these files into .mp4 so that I can put them onto my iPhone, but my iPhone cannot access the audio tracks while watching the episode, so if the default track is in japanese I can't watch the episode at all. I'm wondering if I'm able to go into each individual episode and set their default track value to the english one so that when I convert them it'll be in english for sure.