[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/search/processors/online.py

@@ -196,16 +196,6 @@ class OnlineProcessor(EngineProcessor):
                 'test': ['unique_results'],
             }
 
-        if getattr(self.engine, 'supported_languages', []):
-            tests['lang_fr'] = {
-                'matrix': {'query': 'paris', 'lang': 'fr'},
-                'result_container': ['not_empty', ('has_language', 'fr')],
-            }
-            tests['lang_en'] = {
-                'matrix': {'query': 'paris', 'lang': 'en'},
-                'result_container': ['not_empty', ('has_language', 'en')],
-            }
-
         if getattr(self.engine, 'safesearch', False):
             tests['safesearch'] = {'matrix': {'query': 'porn', 'safesearch': (0, 2)}, 'test': ['unique_results']}