I found two bugs:
1) If you enable the "expect completed on ..." option in activity completion for an activity, then the statement "should be completed" will be shown in the upcoming events. This statement has no string identifier. If you add ?strings=1 to the URL, you will see no string identifier.
2) In the fa language pack there is a translation for "core_completion / completionexpectedfor" string identifier. I think this should be the string identifier for the above statement, but it is not applied when you change the language pack to fa.