Under Server go to Languages and set Subtitle Mode to Always Enabled and choose prefered subtitle language.
For web player, it works.
For Ios player, it doesn’t work, I have to set subtilte for every episode. Please fix it, or could anyone tell me how to make it work?
On the iOS app, sign out of your plex.tv account, restart, and sign back in.
Or Switch User if you have another user in your Plex Home and switch back.
Either of the above will force the client to load again your language preferences from your account.