Hi @Nicolas_SCHUB ,
I am afraid this is an unsupported use-case.
Also I am not sure whether it is actually possible to support this with the app as it is implemented at the moment. What this app basically does is streaming the actual media file from your plex server, rather than acting as a Plex Player. So it is oblivious to what you have configured in plex itself and will use the defaults of the media file instead.
In the codebase I currently have no options to retrieve or control language and subtitle.
It might be possible if I upgrade the api to an actual plex player. But this is a very big maybe and cannot happen any time soon.
Sorry about that.
Thank you for your answer 
Suggesting for the FAQ on a problem (with fix) that i encountered.
My plex instance was showing offline, this was caused because the host where plex is running has multiple ip adresses. In the network settings of plex there is a option called “Preferred network interface“ which despite having the setting on “both“ could not connect with this app. When i changed it to a fixxed preferred ip the connection was good and the app is working.
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Thanks @James_Woudenberg
Added it to FAQ.
