I've made a Dutch translation for the DataObject_Manager.
I also removed some typo's from and added the right filename to the Dutch translationfile of the ImageGallery.
BTW The "Add images to..." button on the Photos tab of the Image Gallery isn't completely translatable. The tag for 'to' is missing. I know you already have one in the DOM-language-file: $lang['nl_NL']['DataObjectManager']['TO'] = 'tot';. But unfortunatly it can't be reused. The 'to' on the button should (in dutch) be translated as 'aan' instead of 'tot'.