I released a new module for Drupal a couple days ago called Internationalization 404. This module helps you automatically use Content Translation-translated localized language versions of your 404, 403, and home pages.
Normally, you have to install Internationalization Variables (i18n_variable) and separately configure the options for these pages per-language. This module saves you time and automatically sends visitors to the correct page. Should you decide to use specific language versions with i18n_variable, this module will respect that configuration and not interfere.
All you have to do is enable the module for it to start working. No configuration required. Disable it to turn off the functionality.
Install it from the project page.
Thanks to Project Ricochet for sponsoring this work.