Force a lingua property for all documents

[gflohr]

The default value for config.linguas should not be the empty list but [en]. When creating assets,
its lingua property should default to the main language of the site, that is the first element of config.linguas. This will ensure that all l-methods of the qgoda plug-in will automatically work.

Exception: The user explicitely sets config.linguas to the empty list. But that should be respected then.