Translation Fixes & Field Based Translation of csv files

Hi Fran,
This pull request contains my previous pull request also:
Following things have been done:
* Radio button instead to check box for "Export to xls"
* File name is <language>.xls rather than trans.xls
* Strings from s3survey were displayed because of some assumptions when considering files starting with s3. It is fixed now.

Previous pull request:
* For csv files earlier only name field was getting translated now list of fields to be translated is obtained from represent.fields.
* It is done for org_sector