Add es-AR conventions and translations i18n files

Cool :)

It has yet to be discussed where and how we want to provide additional locales and translations. We did not want to decide about this matter.

Bundling these with Rails core has the disadvantage that only some languages would work with the Simple backend provided by the I18n gem, while others need more sophisticated means for, e.g. pluralization etc.

Our current thinking is to have a central place where these things can be collected and installed from separately.

I'm working on a plugin that offers some of these options and can be configured using a YAML configuration file. Look at my GitHub repository in the next few days - it should be up by then if I can manage to package it up in a usable form.