How do I render a video with 2 languages, and have a video player like VLC choose between 2 audio tracks (i.e. 1 track for Japanese and the other for English)?
You can just use MKVToolNix to add the second track to the MKV file after rendering, it's still another step but doesn't require re-encoding.
If you're just trying to multiplex tracks and not actually edit the video, I'd recommend doing it entirely with MKVToolNix and skipping Kdenlive for this use. I've done this previously to combine a subbed video and a dubbed one into one file, you can offset or stretch the audio if needed as well.
You can just use MKVToolNix to add the second track to the MKV file after rendering, it's still another step but doesn't require re-encoding.
If you're just trying to multiplex tracks and not actually edit the video, I'd recommend doing it entirely with MKVToolNix and skipping Kdenlive for this use. I've done this previously to combine a subbed video and a dubbed one into one file, you can offset or stretch the audio if needed as well.