Add a banner for outdated translations to invite new translators

Currently at least Persian, Polish, Turkish and Chinese (traditional) translations are pretty outdated. This pull request adds a banner to these languages in order to invite new translators.

![capture du 2014-08-03 15 17 15](https://cloud.githubusercontent.com/assets/3578089/3793136/b6436fa4-1b8c-11e4-9a77-951f22eb5915.png)