I18n layer, introduced in Rails 2.2, is said to be “the simplest thing that ever could work”. Too bad they didn’t go for “the simplest thing that could be practical to use”. These two can be very far apart. I have a hard time imagining a real-world app using only bare Rails’ I18n engine.
Another unpleasant thing is that Rails 2.2 broke GetText compatibility, which prevents many projects from migrating to Rails 2.2 or later versions.