fix(giscus): fallback to en if no language code is set (#1084)

Author: CaiJimmy

layouts/partials/comments/provider/giscus.html

@@ -11,7 +11,7 @@
     data-emit-metadata="{{- default 0 .emitMetadata -}}"
     data-input-position="{{- default `top` .inputPosition -}}"
     data-theme="{{- default `light` .lightTheme -}}"
-    data-lang="{{- default $.Language.LanguageCode .lang -}}"
+    data-lang="{{- default (default `en` $.Language.LanguageCode) .lang -}}"
     data-loading="{{- .loading -}}"
     crossorigin="anonymous"
     async