fix lang_COUNTRY not fallback correctly for IndexBuilder

In IndexBuilder, it will fallback to <lang> when the lang
is passed as <lang_COUNTRY>, and will fallback to en when
not found.

::
    lang_class = languages.get(lang)    # type: Type[SearchLanguage]
                                        # add language-specific SearchLanguage instance

    # fallback; try again with language-code
    if lang_class is None and '_' in lang:
        lang_class = languages.get(lang.split('_')[0])

    if lang_class is None:
        self.lang = SearchEnglish(options)  # type: SearchLanguage

So there's no need to check lang before creating the IndexBuilder
instance.

Signed-off-by: Shengjing Zhu <i@zhsj.me>

CHANGES

@@ -103,6 +103,7 @@ Bugs fixed
 
 * i18n: message catalogs were reset on each initialization
 * #4850: latex: footnote inside footnote was not rendered
+* i18n: fix lang_COUNTRY not fallback correctly for IndexBuilder
 
 Testing
 --------

sphinx/builders/html.py

@@ -444,9 +444,9 @@ class StandaloneHTMLBuilder(Builder):
         # create the search indexer
         self.indexer = None
         if self.search:
-            from sphinx.search import IndexBuilder, languages
+            from sphinx.search import IndexBuilder
             lang = self.config.html_search_language or self.config.language
-            if not lang or lang not in languages:
+            if not lang:
                 lang = 'en'
             self.indexer = IndexBuilder(self.env, lang,
                                         self.config.html_search_options,