So, everything works fine locally, but at heroku it changes to :en
Fire up console on both environments:
Heroku:
heroku console --app myapp
It helped setting locale directly.
config.i18n.default_locale = :nb #Adding the below makes it work as expected at heroku config.i18n.locale = :nb
If you have a better solution, please let me know and you'll get upvote and accepted answer.