en is Indian English — English words in an Indian accent. It is not
British or American, and there is no code for those.
A wrong code is not an error. language must match the script your text
is actually written in. Hindi text with "language":"ta" is accepted and
pronounced wrongly, with no error at all. Nothing warns you — you find out by
listening.
Code-mixed text
Leave language out when your text switches languages mid-sentence:
Auto-detection handles the mixture. Pinning a language forces every word through
one pronunciation model, including the ones that do not belong to it. 
