Following this blog article I enabled my application to load i18n messages from the database. It works great. However, I don\'t want to manage all messages in the database
I strongly encourage you to use Copycopter
(http://copycopter.com) to manage i18n keys content. It will allow your team to update keys on the fly easily, using a friendly interface, and you will be able to set the default as you wanted. This way no one will come to you when a key needs to be updated.
ThoughtBot, a strong web development consulting company is behind that product. It is solid.
Your code will look like this then:
mail :to => member.email,
:subject => I18n.translate("member_mailer.reminder_to_sign_in.#{underscored_number_of_days}.subject",
:default => %{You have not signed in for #{humanized_number_of_days}})
The i18n keys are generated on the fly on copytcopter, the first time you use it. It is really easy to use.
Check it out, it is worth it.