[fix] replace language_support by a language/region view

Most engines response best results if a region is selected, most often a
language is also in the properties of a engine and sometimes the language
argument is just the language of the UI.  Most often choosing a language has a
minor effect on the result list.

To summarize:

Some engines have language codes (e.g. `ca`) in their properties, some have
region codes (e.g. `ca-ES`), some have regions and languages in their properties
and other engine do not have any language or region support.

In the past we generalized *language* over all kind of engines without taking
into mind that most engines gave best result when there is a region selected.

  This *language-centric* view in SearXNG is misleading when we need
  region-codes to parameterize engine request!

This patch replaces the *language-centric* view by a "language / region" view.

Conclusions:

With regions we can't say any longer that a engine supports *this or that*
language, by example: when the user selects 'zh' and a engine supports only
region codes like 'zh-TW' or 'zh-CN' we do not what results the user expects /
similar with 'en' or 'fr when the engine needs a region tag.

- Since it is unclear what the user expects by his language selection, we can't
  assert a property that says: "supports_selected_language"

  The feature is replaced in the UI by the wider sense of "language_support",
  what stands for:

    The engine has some kind of language support, either
    by a region tag or by a language tag.

- A list of "supported_languages" does not make sense when there are regions
  responsible for the result of an engine.

  The "supported_languages" has been removed from the /config URL

- The `has_language` test in the `searx/search/checker/impl.py` has been removed
  since it does not cover engines with region support.

  If there is a need for such a test we can implement new tests after all
  engines with language (region) support has been moved to the *supported
  properites* scheme (see searxng_extra/update/update_languages.py)

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

searx/templates/oscar/preferences.html

@@ -344,7 +344,6 @@
                                     <th scope="col">{{ _("Allow") }}</th>
                                     <th scope="col">{{ _("Engine name") }}</th>
                                     <th scope="col">{{ _("Shortcut") }}</th>
-                                    <th scope="col" class="col-stat">{{ _("Selected language") }}</th>
                                     <th scope="col" class="col-stat">{{ _("SafeSearch") }}</th>
                                     <th scope="col" class="col-stat">{{ _("Time range") }}</th>
                                     <th scope="col">{{ _("Response time") }}</th>
@@ -356,7 +355,6 @@
                                     <th scope="col" class="text-right">{{ _("Response time") }}</th>
                                     <th scope="col" class="text-right">{{ _("Time range") }}</th>
                                     <th scope="col" class="text-right">{{ _("SafeSearch") }}</th>
-                                    <th scope="col" class="text-right">{{ _("Selected language") }}</th>
                                     <th scope="col" class="text-right">{{ _("Shortcut") }}</th>
                                     <th scope="col" class="text-right">{{ _("Engine name") }}</th>
                                     <th scope="col" class="text-right">{{ _("Allow") }}</th>
@@ -383,7 +381,6 @@
                                             {{- engine_about(search_engine, 'tooltip_' + categ + '_' + search_engine.name) -}}
                                         </th>
                                         <td class="name">{{ shortcuts[search_engine.name] }}</td>
-                                        <td>{{ support_toggle(supports[search_engine.name]['supports_selected_language']) }}</td>
                                         <td>{{ support_toggle(supports[search_engine.name]['safesearch']) }}</td>
                                         <td>{{ support_toggle(supports[search_engine.name]['time_range_support']) }}</td>
                                         {{ engine_time(search_engine.name, 'text-right') }}
@@ -395,7 +392,6 @@
                                         {{ engine_time(search_engine.name, 'text-left') }}
                                         <td>{{ support_toggle(supports[search_engine.name]['time_range_support']) }}</td>
                                         <td>{{ support_toggle(supports[search_engine.name]['safesearch']) }}</td>
-                                        <td>{{ support_toggle(supports[search_engine.name]['supports_selected_language']) }}</td>
                                         <td>{{ shortcuts[search_engine.name] }}</td>
                                         <th scope="row" data-engine-name="{{ search_engine.name }}"><span>{% if search_engine.enable_http %}{{ icon('exclamation-sign', 'No HTTPS') }}{% endif %}{{ search_engine.name }}</span>{{ engine_about(search_engine) }}</th>
                                         <td class="onoff-checkbox">