Hey Sipke,
Was about to translate, but checked the forum first. Just signed up to thank you: great job!
Hope this will be included in the next version because I've got a few older people to use KeePassX. They are not good at English, so it's a bit troublesome sometimes.

Edit:
I've checked the translation, it is VERY good! Some interesting points, not improvements, but just the way I look at them.
This translation can be added as-is imho. Expired is translated as "Vervallen" now, wouldn't "Verlopen" be more accurate? Or am I just lost in convention?
"Onverwachte fout: 'Offset is out of range'." is a difficult one.
I've found this online:
EN: offset, key offset NL: sleuteloffset
Definitie: (Engels) a) result of adding a counter to a cryptographic key using modulo-2 addition; b) number that mathematically relates a calculated identification code to a cardholder PIN; the process of modulo-2 adding a counter to a key
So "Onverwachte fout: 'Sleuteloffset is buiten bereik'." could be an alternative.
Looking at the rest of the keys, sleuteloffset could be a good guess.
Maybe a programmer can shed some light on this, what exactly is meant by this line in this context?"Reguliere Expressie" is a dutch term, so it can be added for "Regular Expression". Although ppl who use it will know the English term. Maybe there will be Dutch speaking ppl that will search for "Regular expression" and get lost?
"De map waar opslagmedia zoals cd's en geheugenkaarten normaliter te vinden (mounted) zijn." I think mounted is usually translated as "gekoppeld". So "De map waar opslagmedia zoals cd's en geheugenkaarten normaliter gekoppeld worden."?
Show plain text passwords in: -> Wachtwoorden voluit tonen in:
Could also be "Wachtwoorden leesbaar tonen in:" of "Wachtwoorden als text tonen in:"