The move comes after, apparently, low usage. From the YouTube help section:
Turning off the Translation marketplace
We’re working to develop tools that benefit the broadest number of creators, such as a our free translation tools. We noticed that usage for the Translation marketplace has been low, so we’ve made the decision to turn it off starting in June 2017.
This means that you’ll no longer be able to buy transcriptions and translations for your videos directly in Video Manager. Don’t worry — If you’ve already purchased translations from the marketplace, they’ll continue to show on your videos. And any orders you place before the marketplace is turned off will still be delivered.