Revert "remove 'all' option from search languages"

This reverts commit 4d1770398a.

searx/engines/wikipedia.py

@@ -31,6 +31,9 @@ supported_languages_url = 'https://meta.wikimedia.org/wiki/List_of_Wikipedias'
 
 # set language in base_url
 def url_lang(lang):
+    lang_pre = lang.split('-')[0]
+    if lang_pre == 'all' or lang_pre not in supported_languages:
+        return 'en'
     return match_language(lang, supported_languages, language_aliases).split('-')[0]