This problem will go away starting with 3.4.0, where we’ve renamed es_ES to es and configured the upgrade to migrate es_AR data over to es as well. In the meantime, I can think of two solutions…
The lowest-fuss solution might actually be to copy the es_ES files over to es_AR and continue to operate with es_AR configured.
Alternately, try taking a database dump and searching it for es_AR. That’ll tell you what parts of the system still have es_AR configured.
Of course, make a good backup before working with the database directly!
Regards,
Alec Smecher
Public Knowledge Project Team