Reminder to myself: It is worth the effort of installing Drupal straight away in the language that the admin users later will be using. Even if you can change the default language straight away after installation it still leads to a weird mix of some modules taking the default language from what was default during installation and some of what currently is default.
And while it might be cumbersome for me to come up with the right words in other languages in menus, terms etc - it is those admins and editors who will spend much more time using the site than me in future.
For languages like Dutch, Spanish or French with more than 99% of core translated - there is a pretty good starting point. However, the Swahili and Creole translations which I just checked for some projects are still below 1%.
However - as others have pointed out before - while the installer gives instructions where to get the po-files to install Drupal 6 in another language, it doesn't explain what to do with this file.
- Download the po-file for your language and version from http://localize.drupal.org/ i.e. drupal-6.22.nl.po
- Rename the file drupal-6.22.nl.po. to nl.po. Other manuals use other names, but I found that didn't work for me.
- Create a translations folder inside /profiles/default/
- Copy the file drupal-6.20.es.po inside /profiles/default/translations
Refresh the installer page and you should have the right language.