After looking around for some time I have found a tip in an Italian site that shows how to turn on all the extra languages that MacOS X PB has inside.
Below I post the tip but read the article for all the extra information:
http://macprof.macity.it/archives/Xinternational.html
<b>How To do it:</b><br>
Use Sherlock to find a file named "<b>.Globa Preferences.plist</b>" in the partition where you have installed Mac OS X.<br>
Open it with a text editor (Word can do the job but a simpler one to open and save simple text files)<br>
Check the first lines...</p>
<p><key>AppleLanguages</key><br>
<array><br>
<string>French</string><br>
<string>English</string><br>
<string>German</string><br>
</array></p>
<p>You can find that only English, German and French are listed... <br>
to support your language simply add these few lines...<br></p>
<p><key>AppleLanguages</key><br>
<array><br>
<string>Spanish</string><br>
<string>Dutch</string><br>
<string>Swedish</string><br>
<string>Italian</string><br>
<string>French</string><br>
<string>English</string><br>
<string>German</string><br>
</array></p>
<p>Change it to add your language (the list contains the ones we have checked....) and then save the update file as a text only archive in its original place.</p>
<p>Now restart your MacOS X beta and login... go to system preferences and choose "international" than put the preferred one (you'll see the updated list that adds more to the 3 standard ones) on top of the list.</p>
<p><br>Log out from MacOS X and log in again.<br>
Now you have a "nationalized" Mac OS X beta: a promise of what the future will bring to you...</p>
<p>Be careful and return to the standard languages to check the full compatibility of your applications and utilities.... and remember once again that the additional language aren't supported by Apple for now...</p>