Nukeador asked on the L10n mailinglist to please create a combined "es" locale instead of using "es_ES" as the localizers collaborate on a combined translation. Once this is available on product-details (bug 580547), we can switch this on Input. Need to fix this in the reporter code, as well as on SVN and in Verbatim.
Should be fixed on staging: http://github.com/fwenzel/reporter/commit/631e78c
Verified FIXED: Tested with Firefox 4 beta 3 es-ES, which redirected on staging to http://input.stage.mozilla.com/es/ from root, and also from /es-ES to /es, manually.
Verified with Minefield es-AR , thanks!