Hi Bjornat, I posted this in response to your question at my blog, it should help:
Explicitly set a language for WordPress. Browse the directory that contains all of files used by WordPress. Edit wp-config.php to change the bit that reads
* WordPress Localized Language, defaults to English.
* Change this to localize WordPress. A corresponding MO file for the chosen
* language must be installed to wp-content/languages. For example, install
* de.mo to wp-content/languages and set WPLANG to 'de' to enable German
* language support.
define ('WPLANG', '');
Change the bottom line to
define ('WPLANG', 'en');
define ('WPLANG', 'es');
or to reflect whatever language files you have installed in "wp-content/languages". The bit that specifies the language used by WP is 'en' (for English) or 'es' (Spanish).
If wp-content/languages doesn't exit (which it will do because you've correctly installed Kau Boy's Backend Localization), create it. Get your language packs from http://svn.automattic.com/wordpress-i18n.
Now try to install Jetpack again. It's safe to revert "define ('WPLANG', 'es');" back to "define ('WPLANG', '');" once Jetpack is installed. Make sure it correctly reflects the backend language chosen for Kau Boy's plugin.