So, I’m back with my findings so far. I have downgraded to the previous version of the media server (meaning 1.23.3.4707) and I can confirm that there are no issues anymore, as previously stated by me. The differences between the two versions can be seen in the below attached screenshots. I want to mention that I have not converted the subtitle file and Is the same on both versions of the media server.
Maybe this is not a bug, but a feature? 
It clearly is a regression… somewhere. I also checked the latest release notes: Plex Media Server - #455 by emilybersin and what triggered me is this new thing that was implemented:
(Localization) Improved language code support for audio and subtitle streams (#12909)
So… this needs to be investigated from my point of view. Till then, I recommend all romanian users to rollback to the 1.23.3.4707 version. 
