I have created a new feature (Giving time/date extensions to individual for course activities specially for assignment module) . I am going to submit patch for this. As per my information the display strings must be included form the lang pack without using them implicitly. How to add this display string in lang, considering en lang pack and "people who will upgrade to use this feature".
see the attachments provided in tracker of this issue. http://tracker.moodle.org/browse/MDL-7315
Thanks...