[mod] add locale.get_engine_locale to get predictable results

The match_language function sometimes returns incorrect results which is why a
new function get_engine_locale is required.

A bugfix of the match_language is not easily possible, because there is almost
no documentation for it and already the call parameters are undefined.  E.g. the
function processes values like the ones from yahoo::

    "yahoo": [
        "ar",
        ...
        "zh_chs",
        "zh_cht"
     ]

The get_engine_locale has been documented in detail, there is a clear
description of the assumptions as well as the requirements and approximation
rules (read doc-string for more details)::

    Argument ``engine_locales`` is a python dict that maps *SearXNG locales* to
    corresponding *engine locales*:

      <engine>: {
          # SearXNG string : engine-string
          'ca-ES'          : 'ca_ES',
          'fr-BE'          : 'fr_BE',
          'fr-CA'          : 'fr_CA',
          'fr-CH'          : 'fr_CH',
          'fr'             : 'fr_FR',
          ...
          'pl-PL'          : 'pl_PL',
          'pt-PT'          : 'pt_PT'
      }

    .. hint::

       The *SearXNG locale* string has to be known by babel!

In the following you will find a comparison:

>>> import babel.languages
>>> from searx.utils import match_language
>>> from searx.locales import get_engine_locale

Assume we have an engine that supports the follwoing locales:

>>> lang_list = {
...     "zh-CN": "zh_CN",
...     "zh-HK": "zh_HK",
...     "nl-BE": "nl_BE",
...     "fr-CA": "fr_CA",
... }

Assumption:

  A. When a user selects a language the results should be optimized according to
     the selected language.

  B. When user selects a language and a territory the results should be
     optimized with first priority on territory and second on language.

----

Example: (Assumption A.)

  A user selects region 'zh-TW' which should end in zh_HK

hint:
  CN is 'Hans' and HK ('Hant') fits better to TW ('Hant')

>>> get_engine_locale('zh-TW', lang_list)
'zh_HK'
>>> lang_list[match_language('zh-TW', lang_list)]
'zh_CN'

----

Example: (Assumption A.)

  A user selects only the language 'zh' which should end in CN

>>> get_engine_locale('zh', lang_list)
'zh_CN'
>>> lang_list[match_language('zh', lang_list)]
'zh_CN'

----

Example: (Assumption B.)

  A user selects region 'fr-BE' which should end in nl-BE

hint:
  priority should be on the territory the user selected.  If the user
  prefers 'fr' he will select 'fr' without a region tag.

>>> get_engine_locale('fr-BE', lang_list, default='unknown')
'nl_BE'
>>> match_language('fr-BE', lang_list, fallback='unknown')
'fr-CA'

----

Example: (Assumption A.)

  A user selects only the language 'fr' which should end in fr_CA

>>> get_engine_locale('fr', lang_list)
'fr_CA'
>>> lang_list[match_language('fr', lang_list)]
'fr_CA'

----

The difference in priority on the territory is best shown with a engine that
supports the following locales:

>>> lang_list = {
...     "fr-FR": "fr_FR",
...     "fr-CA": "fr_CA",
...     "en-GB": "en_GB",
...     "nl-BE": "nl_BE",
... }

----

Example: (Assumption A.)

   A user selects only a language

>>> get_engine_locale('en', lang_list)
'en_GB'
>>> match_language('en', lang_list)
'en-GB'

hint: the engine supports fr_FR and fr_CA since no territory is given, fr_FR
takes priority ..

>>> get_engine_locale('fr', lang_list)
'fr_FR'
>>> lang_list[match_language('fr', lang_list)]
'fr_FR'

----

Example: (Assumption B.)

  A user selects region 'fr-BE' which should end in nl-BE

>>> get_engine_locale('fr-BE', lang_list)
'nl_BE'
>>> lang_list[match_language('fr-BE', lang_list)]
'fr_FR'

----

If the user selects a language and there are two locales like the following:

>>> lang_list = {
...      "fr-BE": "fr_BE",
...      "fr-CH": "fr_CH",
...  }
>>>

>>> get_engine_locale('fr', lang_list)
'fr_BE'
>>> lang_list[match_language('fr', lang_list)]
'fr_BE'

Looks like both functions return the same value, but match_language depends on the
order of the dictionary (which is not predictable):

>>> lang_list = {
...      "fr-CH": "fr_CH",
...      "fr-BE": "fr_BE",
...  }
>>> get_engine_locale('fr', lang_list)
'fr_BE'
>>> lang_list[match_language('fr', lang_list)]
'fr_CH'
>>>

The get_engine_locale selects the locale by looking at the "population percent"
and this percentage has an higher amount in BE (68.%) compared to CH (21%)

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>