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.
Nicolas Gelot