Sitemap hreflang syntax invalid for regional language variants
The `hreflang` for regional variations in `sitemap.xml` follows the format of the URL language slug [generated by Sphinx](http://www.sphinx-doc.org/en/master/usage/configuration.html#confval-language), e.g. `zh_CN` for Chinese (China). This is invalid syntax for `hreflang` in a sitemap ([reference](https://en.wikipedia.org/wiki/Hreflang#Common_Mistakes)), where a hyphen must be used instead, e.g. `zh-CN`. Alternatively, it is also valid to define the script of the language instead, e.g. `zh-Hans` for Simplified Chinese. Some sort of library to perform this conversion, a lookup table or a user-facing option could help fix this.

Example from: https://docs.dash.org/sitemap.xml
```
<xhtml:link rel="alternate" hreflang="zh_CN" href="http://docs.dash.org/zh_CN/latest/"/>
```

diff --git a/readthedocs/core/views/serve.py b/readthedocs/core/views/serve.py
index cefbb7b50..6460a1002 100644
--- a/readthedocs/core/views/serve.py
+++ b/readthedocs/core/views/serve.py
@@ -356,6 +356,17 @@ def sitemap_xml(request, project):
         priorities = [1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2]
         yield from itertools.chain(priorities, itertools.repeat(0.1))
 
+    def hreflang_formatter(lang):
+        """
+        sitemap hreflang should follow correct format.
+
+        Use hyphen instead of underscore in language and country value.
+        ref: https://en.wikipedia.org/wiki/Hreflang#Common_Mistakes
+        """
+        if '_' in lang:
+            return lang.replace("_", "-")
+        return lang
+
     def changefreqs_generator():
         """
         Generator returning ``changefreq`` needed by sitemap.xml.
@@ -409,7 +420,7 @@ def sitemap_xml(request, project):
                         private=False,
                     )
                     element['languages'].append({
-                        'hreflang': translation.language,
+                        'hreflang': hreflang_formatter(translation.language),
                         'href': href,
                     })
 
diff --git a/readthedocs/rtd_tests/tests/test_doc_serving.py b/readthedocs/rtd_tests/tests/test_doc_serving.py
index d2ed7adf1..a1ac0d0f9 100644
--- a/readthedocs/rtd_tests/tests/test_doc_serving.py
+++ b/readthedocs/rtd_tests/tests/test_doc_serving.py
@@ -250,6 +250,13 @@ class TestPublicDocs(BaseDocServing):
             main_language_project=self.public,
             language='translation-es'
         )
+        # sitemap hreflang should follow correct format.
+        # ref: https://en.wikipedia.org/wiki/Hreflang#Common_Mistakes
+        hreflang_test_translation_project = fixture.get(
+            Project,
+            main_language_project=self.public,
+            language='zh_CN'
+        )
         response = self.client.get(
             reverse('sitemap_xml'),
             HTTP_HOST='public.readthedocs.io',
@@ -287,3 +294,6 @@ class TestPublicDocs(BaseDocServing):
                 private=False,
             ),
         )
+        # hreflang should use hyphen instead of underscore
+        # in language and country value. (zh_CN should be zh-CN)
+        self.assertContains(response, 'zh-CN')
['readthedocs/core/views/serve.py', 'readthedocs/rtd_tests/tests/test_doc_serving.py']