Default language of document is not preserved on publication

Steps to reproduce:

• create a document with default language -> normally this is happening on recent wikis if you have a wiki which has a default language set (e.g. fr)
• publish the document. The published document will not have default language set anymore. The value will be empty.

This is particularly bothering if solr is used to search in the published documents, as they won't have a set language so they won't be stemmed properly in the declared language of the wiki and the searched will be crippled.