feat(Multilingual Unit): Log selected language and load previous-selected language on VLE start

[hirokiterashima]

Changes

• When in student run mode
  • Log language selection events in the event table. Every time the student switches a language, we save an event.
  • Display unit in previously-selected language, if applicable

Test

• In a student run for a unit with multiple languages
  • switch between different languages on different steps. In the database, check that the events are saved in the events table.
    • The event name is "languageSelected"
    • The event data is {"requester":"user","language":"[LOCALE]"}
    • You should also see a row for {"requester":"system","language":"[LOCALE]"}. This is saved when the VLE loads, it looks up the last-saved language in StudentStatus and switches to it
  • Refresh the browser, or exit -> reload the unit. The unit should be rendered in your last-saved language.
• In a student run for a unit without multiple languages, everything works as before

[breity]

Looks good. 👍

[geoffreykwan]

Looks good.