I figured out that Magento creates a file called 'js-translation.json' in the static folder. If I edit this file directly, I see that my translations are updated. But where is this file created, and how can I add translations so that Magento will automatically add this to js-translation.json when it's generated?
The correct answer to this question is still not given, altough Mage2.PRO pointed me to the right direction, it's not the full answer. My js-translation.json is still empty when generated.
For example, I like to translate the Add To Cart button. This button has three states, Add To Cart, Adding... and Added... How can I translate these words? Another example, the 'You have no items in your shopping cart.' text in the minicart. I think all these phrases should be translated from js-translation.json, but where to add them so these file is filled with these translations?