First of all, I think it's still a great translation, although I did see a 'typo'. This translation makes sense in a accounting way of speaking. It's not a nonsense dictionary translation. Great job !!
About the php-file (I do use PoEdit, but couldn't find a way to create a new PHP file)
First thing I did is deleting lines 3-9. Because of the sess*-files bug, these lines are making the sess*-files not just big, but huge
'' => 'Seems that system upgrade to version 2.1 has
\t\t\tbeen performed for this company already.<br> If something has gone
\t\t\twrong and you want to retry upgrade process you MUST perform
\t\t\tdatabase restore from last backup file first.Het lijkt erop dat de system update naar versie 2.1 al
is uitgevoerd voor deze administratie.<br>Als er iets mis is gegaan
\t\ten u wilt het opnieuw proberen dan MOET
eerst een database restore uitvoeren van de laatst werkende versie.',
Next error is line 75
'Login timeout must be positive number not less than 10.' => 'De \',
Next error is line 450
'The Retained Earnings Account or the Profit and Loss Year Account has not been set in System and General GL Setup' => 'De rekening voor onverdeelde winst voorgaand boekjaar en/of winst huidig boekjaar is niet ingesteld in het menu \',
Next error is line 743
'You can edit directly only journal entries created via Journal Entry page.' => 'U kunt alleen de journaalposten wijzigen die zijn aangemaakt via het scherm \',
Next error is line 1070
'Deleting GRN items during invoice entry' => 'Verwijder regel \',
Next error is line 1261
'In case of no gettext support, php emulation is used' => 'In geval \',
Next error is line 1281
'\'%s\' file should be read-only' => '\'%s\' bestand moet \',
Next error is line 1283
'\'%s\' file should be read-only if you do not plan to add or change companies' => '\'%s\' bestand moet \',
The final error is line 2824
'Enter Valid until Date' => 'Geef een geldige \',