erp5_core: Display language by default in "Documents" listbox

When we have more than one language and many documents with same reference but with different language, the first impression is that we have many duplicated documents.

Display the language, we can easily detect that we have same reference to different languages.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:36:09

passed ERP5.PerformanceTest-Master

00:26:02

passed ERP5.UnitTest-Master

01:03:23

failed ERP5.UnitTest-Master.Medusa

01:12:23

passed SlapOS.Eggs.UnitTest-Master.Python2

00:19:31

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:41