Change effective on 16 January 2018
The Heroku-16 stack image has been updated. This update is adding the GNU gettext package to the image, at build time only.
The new stack image will be rolled out automatically in the next 24 hours. You do not need to redeploy your app or restart your dynos.
See this Dev Center article for an overview of the libraries available on the Cedar-14 and Heroku-16 stack images.