www.makemkv.com

I can convert my DVDs with the correct spoken language (German) into mkv files well.
I've only the issue, that if the actors in the movie speak foreign languages (e.g. french), that are subtitled with my spoken language (German) playing the DVD - that subtitles are missing in the mkv.
I don't want to subtitle the whole movie - only the part where subtitles are shown in the "real" DVD playback or in the cinemas.

Is there an option in MakeMKV to achieve this?

MakeMKV is going to copy the subtitle tracks as they exist on the DVD. If those tracks include "foreign audio only" or "forced" subtitles, those would be available if you select to include that track.

These could be in their own track (most common), or included in another track with "flags" (not common). It seems that many German disks have a wider variety of subtitle tracks than similar disks for other markets.

I have found that the only way to be certain of what you get is to have MakeMKV include ALL subtitle tracks in the rip, then check the resulting MKV file by playing it in VLC to find which track has what you want.

Once you know which track, you can use a tool like mkvtoolnix header editor to flag that track as "forced" and "default", to cover how players decide which subtitles to use by default. You do not have to remove any tracks, if the flags are set correctly.