Hello there, @mmtscn ...
To be sure, students are able to set their own language settings at the Canvas user account level. @kmeeusen is correct that the language settings would only affect the core Canvas interface and not documents you load into the course. But, I do understand what you are talking about where you're not able to see the correct language on your SpeedGrader screen. For students, they would set their preferred language using the following Guide:
How do I change the language preference in my user account as a student?
Since you are the instructor, there are two areas that you should check to confirm the language is set correct.
Also, this may be a long-shot, but I was looking again at the first screen shot you included in your question. It does seem that part of the text is in Korean, but yet other parts are in English. So, I wonder if somehow the titles of those quiz questions got entered as Korean text? Again, this probably isn't the case, but it might not hurt to check. Go to your quiz, and edit question 1, 2, 3, or 7. Do you see Korean text in the title of the question? Or, is it English text?
If none of this helps resolve the issue, you may need to reach out to Canvas tech support. They may be able to take a look at your course to see if they can spot any abnormalities. How do I get help with Canvas as an instructor? - Canvas Community
I hope this will be of some help to you. Let Community members know if you have any other questions about this...thanks!
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