Datastore depends on english localisation 

There are many checks where we rely on english error messages in the datastore.

See https://github.com/okfn/ckan/blob/master/ckanext/datastore/db.py

It is probably a good idea to use the [error codes](http://www.postgresql.org/docs/9.1/static/errcodes-appendix.html) instead of error message even though one error code seems to subsume a number of possible errors.