To make it short: strftime() does not translate or localize at all.
So in order to make things work in Rails, you need to use I18n localize
I assume you’ve set your I18n in your rails app properly (e.g. with this gem rails-i18n) and the locale is :de
date = Date.new(2017,01,31) l(date, format: "%B %Y") # => Januar 2017
why not read a super detailed blog post from stackednotion.com on this topic?