Dutch: only half the text is translated

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Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL (optional but very useful):http://www.schaatsverenigingzwolle.nl/gallery/
Gallery version:v1.5.5-pl1
Apache version:Apache/1.3.33 (Debian GNU/Linux) mod_layout/3.2 PHP/4.4.4-0.dotdeb.3 with Suhosin-Patch mod_ssl/2.8.22 OpenSSL/0.9.7e
PHP version (don't just say PHP 4, please):PHP Version 4.4.4-0.dotdeb.3
Graphics Toolkit:
Operating system:Windows XP Pro
Web browser/version (if applicable): IE6 & Firefox 1.5

Hi all,

I've been running Gallery for over an year now and just recently discovered something strange.
I always assumed that some of the text was shown in English because the Dutch translations was incomplete. But when I checked the nl_NL-gallery_core.po file I saw that most of the test IS translated.

This is most apparent when adding photos to an album. The texts Click the <b>Browse</b> button to locate a photo to upload. and If no caption is set, what would you like the captions to default to are the only two texts which get translated into Dutch. The other text on the form do not get translated.
My site runs on a shared server, here's the PHP info http://server9.firstfind.nl/ of the server the helpdesk directed me to but I think my site runs on http://server15.firstfind.nl/. I think the setup of these machines are not identical. Can this be the cause of my problems?
Below are the messages I get when runnig setup.

Setup messages:
Dates in 1 languages may not be formatted properly, because the corresponding locales are missing. You may need to select manually the date formats for these.
(Advanced)
System locale problems
There are no apparently suitable system locales installed on your machine for the following languages. Please choose the one you think is most suitable.
This is only for date & time format. You only need to edit the languages you enabled above which shows me the option to either select "System Locale" or "Dutch" but no matter what I select it always returns to "System locale"

Hi Jens,

thanks for your reply but I'm afraid it doesn't solve my problem.

Here's what users see when uploading photo's:

Klik op de Bladeren... knop om de bestanden te selecteren.
After you added a file, you can add more files on the sane way, just use the browse button as often as you want.
Its possible to enter a manual caption for each item you upload, or you can gallery set the captions.
Supported file types: jpg, jpeg, gif, png, asx, asf, avi, mpg, mpeg, mp2, wmv, mov, qt, swf, mp4, rm, ram
You can also upload compressed archive files (zip) full of photos and movies!

Here are the corresponding translations from nl_NL-gallery_core.mo
#: ../includes/add_photos/add_form.inc:31
msgid "Click the <b>Browse</b> button to locate a photo to upload."
msgstr "Klik op de <b>Bladeren...</b> knop om de bestanden te selecteren."This one gets properly translated.

After you added a file, you can add more files on the sane way, just use the browse button as often as you want.
Does not get translated since it's not in the translation.

#: ../includes/add_photos/add_form.inc:33
msgid "Supported file types"
msgstr "Ondersteunde bestandsformaten"

and

#: ../includes/add_photos/add_form.inc:38
msgid "You can also upload compressed archive files (%s) full of photos and movies!"
msgstr "Verstuur een gecomprimeerd bestand (zip, rar, ace, ...) met afbeeldingen en/of filmpjes!"

do not get translated.
So my question still remains; why isn't everything translated?