I upgraded my library to the new Plex Agent and now Plex doesn’t recognise .English srt files and says unknown srt but .English for srt was working before and I don’t want to change all my srt files to .Eng.
The default metadata agents have been rewritten recently.
They are emulating all the documented behaviour of the old agents. The English.srt is undocumented and therefore not part of the new agents.
Yes, you should only use ISO-639-1 (2-letter) or ISO-639-2/B (3-letter) standard language codes (e.g. .en or .eng) – not full-length language names (e.g. .English)!