You appear to have a buggy version of localize_ext. This patch worked for me (but it's fixed - and updated - in the current version of localize_ext):

Index: vendor/plugins/localize_ext/lib/ext/core_ext.rb
===================================================================
--- vendor/plugins/localize_ext/lib/ext/core_ext.rb     (revision 260)
+++ vendor/plugins/localize_ext/lib/ext/core_ext.rb     (working copy)
@@ -54,7 +54,7 @@
   end

   def +(value)
-    return plus(value.to_s) if !value.nil? and value.is_a?(Symbol)
+    return plus_without_localize_ext(value.to_s) if !value.nil? and value.is_a?(Symbol)
     plus_without_localize_ext(value) 
   end