change the Setswana locale from tn_ZA to tn_BW since it's mostly

Botswana anyway according to @sagemaniac in
http://gallery.menalto.com/node/97840

modules/gallery/helpers/locales.php

@@ -103,7 +103,7 @@ private static function _init_language_data() {
     $l["sl_SI"] = "Slovenščina";              // Slovenian
     $l["sr_CS"] = "Srpski";                   // Serbian
     $l["sv_SE"] = "Svenska";                  // Swedish
-    $l["tn_ZA"] = "Setswana";                 // Setswana
+    $l["tn_BW"] = "Setswana";                 // Setswana
     $l["tr_TR"] = "Türkçe";                   // Turkish
     $l["uk_UA"] = "українська";         // Ukrainian
     $l["vi_VN"] = "Tiếng Việt";               // Vietnamese