A document without default language is created when it's created from a file upload from Wysiwyg (CKEditor), despite wiki language settings

Steps to reproduce:

• in the administration of the wiki, set the wiki language to French (fr), no multilanguage
• create a new document
• in document content edit form displayed, before saving the document from the first time, use the CKEditor to upload an image or a file (either by copy-paste of an image or by using the upload menu of the "add image" or "add link to attached file" )
• fill in the content of a document (besides the image/attachment)
• save the document

Expected result:

• the default language of the created document is French (fr)

Actual result:

• the default language of the created document is empty (no language is set)

This issue is particularly bothering because:

• the language of the document impacts the way the document is indexed and thus the quality of the search results - it practice it can make it not appear in the list of search results even if other documents containing the exact same piece of text will appear in the results for a given search string.
• in a single language wiki, there is no method for a regular user or admin (non-developer) to fix the language of this document to make it indexed properly