refactor engine's search language handling

Add match_language function in utils to match any user given
language code with a list of engine's supported languages.

Also add language_aliases dict on each engine to translate
standard language codes into the custom codes used by the engine.

searx/preferences.py

@@ -115,10 +115,6 @@ class SearchLanguageSetting(EnumStringSetting):
                 pass
             elif lang in self.choices:
                 data = lang
-            elif data == 'nb-NO':
-                data = 'no-NO'
-            elif data == 'ar-XA':
-                data = 'ar-SA'
             else:
                 data = self.value
         self.value = data