Revert "Fix wrong HTML lang attribute on non-English pages (#1247)" (#1361)

This reverts commit a54d19e88e.

config.toml

@@ -1,4 +1,5 @@
 baseURL = "https://docs.joinmastodon.org/"
+languageCode = "en-us"
 defaultContentLanguage = "en"
 title = "Mastodon documentation"
 pygmentsCodeFences = true
@@ -190,7 +191,6 @@ disablePathToLower = true
   [languages.en]
     contentDir = "content/en"
     languageName = "English"
-    languageCode = "en-US"
     weight = -99
 
   [languages.pl]
@@ -201,7 +201,6 @@ disablePathToLower = true
   [languages.zh-cn]
     contentDir = "content/zh-cn"
     languageName = "简体中文"
-    languageCode = "zh-CN"
     weight = 1
 
   [languages.ja]