Bug#506616: update-tasks: Improve maintenance and handling of language tasks

[Frans Pop <elendil@planet.nl>]

On Sunday 23 November 2008, Frans Pop wrote:
> At the same time update-tasks should be made to treat desktop-specific
> language tasks the same as the desktop task itself: i.e. only include a
> desktop language task's key packages in the essential list if the
> desktop task itself is "supported" for that task.list.

Note that currently desktop-specific language tasks are not listed at all 
for secondary desktop tasks included in task.list.