Sorry, DSM 7 is still a giant construction site, AFAIK. No experiences so far. Development for Plex on that platform is also still a work-in-progress.
You may want to take a look at this: https://support.plex.tv/articles/204985278-account-audio-subtitle-language-settings/
Of course this will only work if your files are correctly tagged with language information on their audio and subtitle streams.
Further resources: How to remove tag spam and set language in several MKV files at once
Set Audio Language tag MP4/MKV
Solution for Audio: Unknown
