fix fetch_languages for bing

Bing has a list of regions that it supports and some of these regions
may have more than one possible language.

In some cases, like Switzerland, these languages are always shown as
options, so there is no issue. But in other cases, like Andorra, Bing
will only show one language at the time, either the region's default or
the request's language if the latter is supported by that region.

For example, if the HTTP request is in French, Andorra will appear as
fr-AD but if the same page is requested in any other language Andorra
will appear as ca-AD.

This is specially a problem when Bing assumes that the request is in
English because it overrides enough language codes to make several major
languages like Arabic dissappear from the languages.py file.

To avoid that issue, I set the Accept-Language header to a language
that's only supported in one region to hopefully avoid these overrides.