[fix] replace language_support by a language/region view

Many WEB-search engines (e.g. startpage) response best results if a **region**
is selected, most often a language filter is also in the properties of the
WEB-search engine.

hint::

   The **search language** should not be mixed: sometimes the language argument
   is just the language of the UI with none effect on the result list.

To summarize:

Some WEB-search engines have language codes (e.g. `ca`) in their properties,
other have a region codes (e.g. `ca-ES`), some have regions and languages in
their properties (e.g. startpage) 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 several WEB-search engines have best results when there is a
region selected.

  This *language-centric* view in SearXNG is misleading when we need
  region-codes to parameterize a 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 (`'type': 'engine_properties'`) / see commit previous
  commit:

    [mod] engines_languages.json: add new type EngineProperties

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

searx/webapp.py

@@ -997,7 +997,6 @@ def preferences():
             'rate80': rate80,
             'rate95': rate95,
             'warn_timeout': e.timeout > settings['outgoing']['request_timeout'],
-            'supports_selected_language': _is_selected_language_supported(e, request.preferences),
             'result_count': result_count,
         }
     # end of stats
@@ -1048,20 +1047,17 @@ def preferences():
     # supports
     supports = {}
     for _, e in filtered_engines.items():
-        supports_selected_language = _is_selected_language_supported(e, request.preferences)
         safesearch = e.safesearch
         time_range_support = e.time_range_support
         for checker_test_name in checker_results.get(e.name, {}).get('errors', {}):
-            if supports_selected_language and checker_test_name.startswith('lang_'):
-                supports_selected_language = '?'
-            elif safesearch and checker_test_name == 'safesearch':
+            if safesearch and checker_test_name == 'safesearch':
                 safesearch = '?'
             elif time_range_support and checker_test_name == 'time_range':
                 time_range_support = '?'
         supports[e.name] = {
-            'supports_selected_language': supports_selected_language,
             'safesearch': safesearch,
             'time_range_support': time_range_support,
+            'language_support': e.language_support,
         }
 
     return render(
@@ -1095,16 +1091,6 @@ def preferences():
     )
 
 
-def _is_selected_language_supported(engine, preferences: Preferences):  # pylint: disable=redefined-outer-name
-    language = preferences.get_value('language')
-    if language == 'all':
-        return True
-    x = match_language(
-        language, getattr(engine, 'supported_languages', []), getattr(engine, 'language_aliases', {}), None
-    )
-    return bool(x)
-
-
 @app.route('/image_proxy', methods=['GET'])
 def image_proxy():
     # pylint: disable=too-many-return-statements, too-many-branches
@@ -1323,10 +1309,6 @@ def config():
         if not request.preferences.validate_token(engine):
             continue
 
-        supported_languages = engine.supported_languages
-        if isinstance(engine.supported_languages, dict):
-            supported_languages = list(engine.supported_languages.keys())
-
         _engines.append(
             {
                 'name': name,
@@ -1335,7 +1317,6 @@ def config():
                 'enabled': not engine.disabled,
                 'paging': engine.paging,
                 'language_support': engine.language_support,
-                'supported_languages': supported_languages,
                 'safesearch': engine.safesearch,
                 'time_range_support': engine.time_range_support,
                 'timeout': engine.timeout,