[C3] Enable language-picker for resources

[ywarnier]

We have added a language_id field to resource_node and resource_file in #7155, but users don't have the option to set the language through the interface (it is set to the course language by default).

Add a configuration setting (in the language section) to enable a Language field (with a dropdown select of all available languages) in every resource-related form.

• Setting name: resource_multi_language
• Setting title: Enable multilingual resources
• Setting description: Select this option to allow all resource creators to pick a language associated with their content. This will enable document variations and better content indexing, in particular when sharing the same resource in multiple courses or contexts and when using the search index tool.
• Default value: false

In the document variations feature (in documents list when editing from the main URL in multiple-URLs setup), add a language selector. By default, the language in the selector is always set to the course language or (if out of a course) to the user language.

In the document create and edit forms, add a language selector in advanced settings.
Same thing in course description, learning paths (when creating documents) and in exercises.