Allow to define a device language

Problem/Motivation

If the Drupal site is multilingual, it should be possible to define per device in which language the content should be displayed there.

Proposed resolution

Add a language field for devices, set a default value and allow language selection, if multiple languages are available.

Then, when sending the schedule to the device, use the correct language for directly delivered content but also for links that the device will use later to pull updated content.