Index: includes/locale.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/locale.inc,v
retrieving revision 1.174.2.12
diff -u -p -r1.174.2.12 locale.inc
--- includes/locale.inc	4 Mar 2010 00:15:28 -0000	1.174.2.12
+++ includes/locale.inc	15 Mar 2010 09:05:10 -0000
@@ -1287,15 +1287,21 @@ function _locale_import_one_string($op, 
     case 'db-store':
       // We got header information.
       if ($value['msgid'] == '') {
-        $header = _locale_import_parse_header($value['msgstr']);
-
-        // Get the plural formula and update in database.
-        if (isset($header["Plural-Forms"]) && $p = _locale_import_parse_plural_forms($header["Plural-Forms"], $file->filename)) {
-          list($nplurals, $plural) = $p;
-          db_query("UPDATE {languages} SET plurals = %d, formula = '%s' WHERE language = '%s'", $nplurals, $plural, $lang);
-        }
-        else {
-          db_query("UPDATE {languages} SET plurals = %d, formula = '%s' WHERE language = '%s'", 0, '', $lang);
+        $languages = language_list();
+        if (($mode != LOCALE_IMPORT_KEEP) || empty($languages[$lang]->plurals)) {
+          // Since we only need to parse the header if we ought to update the
+          // plural formula, only run this if we don't need to keep existing 
+          // data untouched or if we don't have an existing plural formula.
+          $header = _locale_import_parse_header($value['msgstr']);
+
+          // Get the plural formula and update in database.
+          if (isset($header["Plural-Forms"]) && $p = _locale_import_parse_plural_forms($header["Plural-Forms"], $file->filename)) {
+            list($nplurals, $plural) = $p;
+            db_query("UPDATE {languages} SET plurals = %d, formula = '%s' WHERE language = '%s'", $nplurals, $plural, $lang);
+          }
+          else {
+            db_query("UPDATE {languages} SET plurals = %d, formula = '%s' WHERE language = '%s'", 0, '', $lang);
+          }
         }
         $headerdone = TRUE;
       }