In SilverStripe 2.4, calling i18n::set_default_locale with anything other that en_US causes empty message boxes in the CMS.
To recreate this behaviour (or behavior if you're en_US!), add to _config.php ...
...then log into the CMS (or refresh if already logged in). Edit a page and navigate away without saving. Instead of 'Are you sure you want to navigate away from this page', you get an empty message box.
The reason I want to call i18n::set_default_locale('en_GB') is so new users are automatically assigned the right interface language when they are created via the grid (which only has First Name, Last Name, Email, and Password fields) rather than by clicking 'Add Member'. What's the correct way to set the default language that doesn't have undesirable side effects?
Note: this is about setting the language in the CMS e.g. so people using the CMS see a 'Behaviour' tab not a 'Behavior' tab. This is not about making the resulting site translatable.