As of #1879120: Use Drupal-specific image and link plugins — use core dialogs rather than CKEditor dialogs, containing alterable Drupal forms, Drupal ships with its own "image" and "link" plugins for CKEditor. What we haven't figured out yet, is how to deal with translations.
CKEditor has its own translation system, as does Drupal: Drupal.t().
Drupal.t() only works if any JS it is used in is loaded via PHP: Drupal dynamically adds the translation JS file for a given JS file (see locale.module's hook_js_alter() implementation).
But … Drupal-specific CKEditor plugins are still CKEditor plugins, hence CKEditor is what loads them, not Drupal. Which means the translation files won't be loaded automatically.
So, the question is: how do we deal with this? Pinging Drupal domain experts Gábor Hojtsy & nod_, as well as the CKEditor developers.
Note: non-Drupal-specific CKEditor plugins should of course use CKEditor's translation system.