It looks like it's a bug.
I started to do russian translation today and noticed that some strings do not appear translated whatever I do with them while others do. Further investigation showed that the only translated strings are those that reside in the main plugin file, where plugin entry points are defined.
I researched "addons" plugin and for it it's true, only strings from addons.c are translated.
Maybe some bug with PLUGIN_SET_TRANSLATABLE_INFO macro?
E-gards, Alexander