Found this happening on 4.2 and 4.3dev but earlier versions are also affected.

When a user creates an activity such as assignment or quiz with a start and/or due date, it will show up in the calendar and the upcoming events block. However, when the user changes their language on their profile or for another user with another language set, the entry in the calendar still shows in the original language that was used when the user created the activity.

Steps to reproduce:

1. Add the upcoming events block to a course.
2. Set your language to French.
3. In the course, create an assignment activity named "Assignment 1" and set a due date to a future date.
4. Notice that the entry in the upcoming events block/calendar reads "Assignment 1 doit être effectué".
5. Now change your language in the profile to English (or another language).
6. Notice that the entry in the upcoming events block/calendar still reads "Assignment 1 doit être effectué".
7. For English, it should read "Assignment 1 is due".