How do I convert a video and keep ONLY Japanese audio with English subtitles?

I’ve got some video files for anime in mkv format that I want to convert to webm format. The files have English and Japanese audio tracks but I want to keep only the Japanese audio track and English subtitles in the output files (No English dubs, thank you).

I’m using QWinff to convert my files and I need to know how I can do exactly what I want.

Thanks.