Bug 20082: Correct description of Vietnamese language and add German translation

Bug 20082: Correct description of Vietnamese language and add German translation

Fixes the language description of Vietnamese according to the
bug report and adds the German translation for the language
name.

To test:
- Run the database update
- Verify that the new descriptions Tiếng Việt shows
  in advanced search language pull down
- Install German translations
- Verify that the German translation 'Vietnamesisch'
  shows in the pull down when German is selected.

Signed-off-by: Roch D'Amour <roch.damour@inlibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>