Repro Steps 

1. Make sure that you have an RTL language pack installed
2. Create a course
3. Add users to the course
4. Add some Activities (or Resources)
5. Go to the course settings screen, and in it enable completion tracking for the course.
6. Set what Activities and how to track completion, via Administration > Course Completion > Condition: Activity completion
7. Navigate to Administration > Reports > Activity completion
8. Switch to an RTL user interface

 
Expected Results
As in English, the columns headers should be displayed correctly - the header of each column should be the name of the activity that its participants data is shown in.
 
Actual Results
The columns headers are not displayed correctly. It seems that they are indented incorrectly  - The header of one column is displayed above the neighboring column.
When there are a lot of participants and a lot of columns, this is almost unnoticeable, and because of that, dangerous. The teacher could easily think that a participant completed or didn't completed an activity, while in reality the data is referring to another activity.
 
Problem
Although the issue could be solved by switching to an LTR user interface for a moment, this is far from being ideal. Fixing this issue could give a great boost to the usage of the activity completion mechanism by RTL users.
 
This issue seems non-theme related. It occurs in both system themes - "Boost" and "Classic".
 
I'm not sure, but the current issue could be a regression bug of MDL-40716.