Currently Zanata doesn't allow the addition of locales for which it has no plural form information. Zanata should allow these locales be added, presenting a warning and using a default, no-plurals-allowed tag.
See related bug for solution. See: http://github.com/zanata/zanata/commit/49ce2a7f68690138aa2c124b6a5ebf0cc40ad9dc http://github.com/zanata/zanata/commit/7b992e7a36110a321e41b5902de4e5d5e52e3013