Need help with Forced Narrative Subs

I have a question about forced narrative Subtitles. I would like to implement them in a movie like Netflix do it for example. So I have a movie where a foreign language is spoken sometimes. I have the video file, German and English dub, German and English sub and a German and English forced narrative sub. The way I would like to implement the subs is that I just have the English and German subs and the forced narrative subs always show up automatically based on the audio track. At the moment the forced narrative subs are handled like normal subs. So I have to switch to them manually. Also it would by nice when the forrced nativity subs don’t show up in the selection when watching the Movie.