MKV to MP4 conversion with subtitles

[billj]

Hi Forum,

I'm trying to convert an MKV file with video encoded as MPEG-H H.265 HEVC and subtitles in 2 languages to MP4 with subtitles in one language.
The video and audio converted (very slowly) but I couldn't work out how to include english subtitles.
For info I'm on windows 10, but this is unlikely to be OS specific.

I realise this may be a very basic question, but didn't find the answer by searching the forum. Appreciate it if someone can direct me to the right place