Allow the teacher to force the language for an activity, as well as at course level

To test this, you need a Moodle site with some alternative language packs installed. I don't really want to write a specific test script here. I would rather the tester did some exploratory testing to try to break this feature (and then failed). The new setting is when you create an activity. There should be a new 'Forced langauge' setting under 'Common module settings'. And, that is like the existing 'Forced language' option in the course settings. Those should only appear to users with moodle/course:setforcedlanguage capability. If set for an activity, that takes precedence over the course setting. If a user without the capability edits an activity, the setting is not lost. (I think that other aspects are covered by automated tests: All the web services that the mobile app will need have been updated to include the new setting. A user cannot user backup/restore to get around the moodle/course:setforcedlanguage capability restriction.)