ActiveRecord requires the postgres server to speak English; add this fact to the docs

Author: balpha

docs/DEVELOPER-ADVANCED.md

@@ -5,7 +5,7 @@ to rails, you are likely much better off with our **[Discourse Vagrant Developer
 
 ## First Steps
 
-1. Install and configure PostgreSQL 9.1+
+1. Install and configure PostgreSQL 9.1+. Make sure that the server's messages language is English; this is [required](https://github.com/rails/rails/blob/3006c59bc7a50c925f6b744447f1d94533a64241/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L1140) by the ActiveRecord Postgres adapter.
 2. Install and configure Redis 2+
 3. Install Ruby 1.9.3 and Bundler.
 3. Clone the project.