When will Dutch be ready for mobile?
This is what we expect also.
We do not know any more than you do, but prior experience tells us that Duolingo tends to release updates for the app when there are (in Duolingo's viewpoint) enough tweaks and changes that make it worthwhile.
Duolingo is probably a) expecting the release of new courses soon (to me this seems the most likely, with Irish on the verge of going into beta), which they would also like to make available on the app or b) waiting for the development of a new feature (less likely, I haven't heard anything about this but who knows....).
Thus, we really cannot give you an ETA for Dutch on iOS and Android, not least because we have no insight into what and how Duolingo thinks and we do not know near enough about app development either. :)