Audio and sub settings are per account, not per server. Each user “account” can set the desire setting. This includes all Managed Users aswell.
Click HERE and select the Manage User.
More Info here.
https://support.plex.tv/articles/204985278-account-audio-subtitle-language-settings/
For remotely or remote family/friends accounts you can give them the below.
They will have to set it themselves.
https://app.plex.tv/desktop#!/account/audio-subtitle
