'stale-while-revalidate' test
with custom 'Accept-Language' header value

Fetch time:
Response:
Expecting the following responses:
  1. Hallo Welt! (timestamp 1)
  2. Hallo Welt! (timestamp 1)
  3. Hallo Welt! (timestamp 2)
  4. Hallo Welt! (timestamp 3)