Ruby on Rails + Devise + I18n: how to set locale?

Take a look at Devise Wiki https://github.com/plataformatec/devise/wiki/I18n They have lots of YML file samples.

If you still wanna write your own, try using something like this in your I18n files

en:
  devise:
    sessions:
      signed_in: 'Signed in successfully.'

More info on GitHub https://github.com/plataformatec/devise#i18n