'Drop component here' message is not translated

When editing a channel, all empty containers show the message 'Drop component here'. That message is not translated.

Steps to reproduce:
1. login to cms.demo.onehippo.com with the Dutch language
2. edit the English Channel

Expected: the empty container on the home page says something like "Sleep componenten hier naartoe" (i.e. the Dutch equivalent of 'Drop component here')
Actual: the empty container on the home page says "Drop component here"