Add array_key_exists check for language in EditEntity api module

when checking multilingual arguments for label or description
changes, 'language' is required in the label or description json
serialization supplied by the user and we try to access
$arg['language'] in the code.  First, we should check that
this key exists and if not, provide an appropriate error response,
in order to avoid php notices and give better error information.

Bug: T76516
Change-Id: If502b2ad917f54a1cc25a8d42fe3b03837d80e50
Formatters for JSON?

A Formatter that outputs JSON is not a Formatter, that's a
Serializer.

Change-Id: Ib91de650f86f65fbb392d30df223b98ca64b59cc
Merge "Formatters for JSON?"
Merge "Add array_key_exists check for language in EditEntity api module"