All right. Going through the most significant bugs in this function:

 * The line from the original function:
     ``langs = request.getHeader('accept-language').split(',')``
   getHeader() returns ``None`` if the header isn't present, so this results
   in a TypeError on the split().

 * The line from the original function:
     ``langs = filter(lambda x: re.match('^[a-z\-]{1,5}', x), langs)``
   This chucks locales with capital letters, and doesn't much at all to insure
   that we're actually getting a well-formed header, all at the expense of a
   (rather expensive; they're slow in Python) regex call.

 * These lines from the original:
       # add fallback languages
       langs_only = filter(lambda x: '-' in x, langs)
       langs.extend(map(lambda x: x.split('-')[0], langs_only))
   If my 'Accept-Language' header starts with 'en-GB,en-US;q=0.92[…]', then
   this would add ['en','en'] to the end of my header, without even checking
   if I already have 'en'. Instead, we should check if 'en' is already there,
   and iff not, then add it *after the other English headers*. Not after
   Mandarin, Japanese, Arabic, Russian, and the other slew of languages that I
   half-assedly learned at some point.

 * These lines from the original:
       # gettext wants _, not -
       map(lambda x: x.replace('-', '_'), langs)
   Great. A pretty mapping. Good thing the returned values weren't saved as
   anything; otherwise they might have been useful!

 * Lastly, the way that languages, once parsed were added to gettext, would
   raise UnhandledErrors, *and* it didn't even add the fallbacks correctly,
   meaning we could only get one language at a time.