Fully implement translation

[kantord]

Right now we have a translation infrastructure, but only the home page is translated, and actually it's always displayed in English, so basically other languages are not used.

This is now needed for courses that are based on other languages, such as French. What is required:

• Allow each course in course.json to force a UI language. This should change the language in the UI automatically to the source language of a course when a course is opened
• In the home page, choosing the language of the UI should be an option available through a dropdown list for example
• Make sure all text in the UI is translatable

[kantord]

So now, translation is possible for the course page, but not for the practice page. The course page is now automatically translated to the "source language" of the course.