So, you want to create your very own translation? Great! You're in the right place. Here's how you can do it:
Where are my files?
The translation files are in your ProjectZomboid folder. Look for a folder like Steam\SteamApps\common\ProjectZomboid\media\lua\shared\Translate. This folder contains every available translation, and the NL folder will be yours.
In it, you'll find what's already been translated (probably from this translation work which stopped at build 32.17). Every file corresponds to a category of translation lines.
How do I know what should be translated?
If you want to know what's missing, you'll have to use the "translation debugger". For that, you first need to modify one of the game executable in order to enable the translation debugger. Do as follows :
Copy either ProjectZomboid32.bat or ProjectZomboid64.bat (according to your system) and open it in a text editor (Notepad++ higly recommended).
Right after the command zombie.gameStates.MainScreenState, leave a blank space and add -debugtranslation
Now, launch the game through this modified file, and once on the main screen, exit. Now, go to the folder user\Zomboid. You should find a translationProblems.txt file. It'll show you every missing line, and in which file of the Steam\SteamApps\common\ProjectZomboid\media\lua\shared\Translate\NL folder it should be added.
You can then copy the missing English line to your NL files and replace the English text by the NL one, using a text editor. Notepad++ is both simple and more attractive than the plain Notepad, so if you don't already have it, give it a try.
The debugger compares the game's current language files to what's in the EN files, then outputs the translationProblems.txt. So... Don't forget to set your languages options to NL first.
Sometimes, you may have remaining lines even though they've been translated. This should be reported to RobertJohnson on the translation bug thread)
How can I make my translation available?
You have two options :
If you're on Steam, create your own Workshop item using the template in user\Zomboid\Workshop (I can give you more details if you need it) and update it at every new build directly from the game. This option allows your fellow NL players to subscribe to your translation and have the files directly updated.
You can also make your files available on hosting platforms like Google Drive or Dropbox. Your users will only need to download the files and put them manually in their ProjectZomboid\media\lua\shared\Translate\NL folder.
Don't forget to post a copy of your files (or a link to where they're hosted) on the Official Translation Files thread, so the devs can add them to the game at every new public release.
You can ask me questions here or via PM if you need more answers