Fix locale and search language

Locale and search language was always defined with english value.

This patch inits the locale on `pre_request` in order to define the
default value of locale and language preferences.

Plus the `best_match` function provided by flask babel library did not
work as expected. So the function `match_language` provided
by searx is used to detect that the language from Accepted-Language
header can be used in searx project.

Close: https://github.com/asciimoo/searx/issues/844
Close: #35, #42