How to add extra language packs to WordPress

I’ve got a multisite which I installed in Dutch.
Therefore English and Dutch are available in the general settings tab.
But I need to add German and French.
I can’t select any other language from the drop down.

And if I add a language in the wp-config.php it just gives a notice without any direction.

Of course I googled but I can’t seem to find anything useful.

How can I active an other language and download the .PO/.MO files?

• http://codex.wordpress.org/WordPress_Localization
• http://codex.wordpress.org/Installing_WordPress_in_Your_Language

1. The easiest way for me is to change language settings for main blog
   multiple times (as mach as many languages I need) and WordPress will
   load these languages automatically.
2. And after that I go to Update page and update language packages (loads languages for
   themes, plugins, etc.)