Add "Auto-detected" as a language.

When the user choose "Auto-detected", the choice remains on the following queries.
The detected language is displayed.

For example "Auto-detected (en)":
* the next query language is going to be auto detected
* for the current query, the detected language is English.

This replace the autodetect_search_language plugin.

searx/search/__init__.py

@@ -3,10 +3,12 @@
 # pylint: disable=missing-module-docstring, too-few-public-methods
 
 import threading
+from copy import copy
 from timeit import default_timer
 from uuid import uuid4
 
 import flask
+import babel
 
 from searx import settings
 from searx.answerers import ask
@@ -20,6 +22,7 @@ from searx.network import initialize as initialize_network, check_network_config
 from searx.metrics import initialize as initialize_metrics, counter_inc, histogram_observe_time
 from searx.search.processors import PROCESSORS, initialize as initialize_processors
 from searx.search.checker import initialize as initialize_checker
+from searx.utils import detect_language
 
 
 logger = logger.getChild('search')
@@ -37,18 +40,57 @@ def initialize(settings_engines=None, enable_checker=False, check_network=False,
         initialize_checker()
 
 
+def replace_auto_language(search_query: SearchQuery):
+    """
+    Do nothing except if `search_query.lang` is "auto".
+    In this case:
+    * the value "auto" is replaced by the detected language of the query.
+      The default value is "all" when no language is detected.
+    * `search_query.locale` is updated accordingly
+
+    Use :py:obj:`searx.utils.detect_language` with `only_search_languages=True` to keep
+    only languages supported by the engines.
+    """
+    if search_query.lang != 'auto':
+        return
+
+    detected_lang = detect_language(search_query.query, threshold=0.0, only_search_languages=True)
+    if detected_lang is None:
+        # fallback to 'all' if no language has been detected
+        search_query.lang = 'all'
+        search_query.locale = None
+        return
+    search_query.lang = detected_lang
+    try:
+        search_query.locale = babel.Locale.parse(search_query.lang)
+    except babel.core.UnknownLocaleError:
+        search_query.locale = None
+
+
 class Search:
     """Search information container"""
 
     __slots__ = "search_query", "result_container", "start_time", "actual_timeout"
 
     def __init__(self, search_query: SearchQuery):
+        """Initialize the Search
+
+        search_query is copied
+        """
         # init vars
         super().__init__()
-        self.search_query = search_query
         self.result_container = ResultContainer()
         self.start_time = None
         self.actual_timeout = None
+        self.search_query = copy(search_query)
+        self.update_search_query(self.search_query)
+
+    def update_search_query(self, search_query: SearchQuery):
+        """Update search_query.
+
+        call replace_auto_language to replace the "auto" language
+        """
+        replace_auto_language(search_query)
 
     def search_external_bang(self):
         """