||LangPackMatcher.jsm calls out to to the addons store, which involves network requests.
Other tests create a fake addons server, and install mock XPIs. At the time of this
writing that infrastructure is not available for mochitests.
Instead, this test mocks out APIs that have a side-effect, so the addons of the browser
are never modified.
The calls to get the app's locale and system's locale are also mocked so that the
different language mismatch scenarios can be run through.
The locales are BCP 47 identifiers: