Bug 6755 Web browser default lang != syspref selected langs

Bug 6755 Web browser default lang != syspref selected langs

When Web browser default selected language doesn't belong to selected
languages list defined by syspref (language / opaclanguages), web pages
are returned in this language, which is wrong...

This patch get this behavior:

- If selected by user language is available (cookie), it is used.
- Otherwise, language is set to default web browser language.
- If this language doesn't belong to languages list (syspref),
  English is selected

Some code cleanup.

To test properly: don't forget to delete your cookies, and to test what
happens without and with cookie.