Lesson: use the right decimal separator in numerical question's feedback page

In MDL-40227 the way decimal separators are handled in lesson was improved. However, the value is not formatted when it's displayed in the feedback page right after submitting an answer.

How to reproduce:

1. In a course, create a Lesson with the following settings:
   1. "Flow control > Provide option to try a question again" set it to "Yes".
   2. "Flow control > Maximum number of attempts per question" set it to "Unlimited".
   3. "Grade > Re-takes allowed" set it to Yes.
2. Add a new question page with type Numerical. Make sure that the right answer has decimals (e.g. "1.1") and add some text in the "Response" of the right answer. 
3. Change your language to one that uses "," as decimal separator (e.g. German).
4. Go to the lesson entry page, you should see the question you just created.
5. Enter the right answer and submit it.
6. In the page that displays the answer and the feedback, check that the value you entered uses "." instead of "," as decimal separator.