Language problem

I have installed your advanced geustbook.
But when i go to admin and want to try to chance the langauge into dutch when i have done that and click on submit settings it chance back to english

I have treid it around 10 times but it keeps on chancing back to english.
Also the geustbook page itself keep on being english.
Also checked out if there was a Dutch langauge file and there is
how can i chance it to Dutch ? and it keeps on being Dutch

Also a other (maybe dumb qeustion)
When run the page i have design thru my localhost server and go strait to the index.php from the geustbook it works out fine, but when i want to let run the whole page and go to the hyperlink for the geustbook index.php of the guestbook shows up as the php script i have treid to just let it go to the folder, i have tried direct linking to the index.php page all is the same i get the php script and not the page what it supposed to be.
What do i do wrong ?
For now (until i put the webpage online)
Running on a localhost apache 2.1
PHP 5.04
Mysql 4.1

Same problem here, started today!