The doc comment spec says that the default value for the `lang` attribute in inline snippets is `java`, and that it is derived from the file name for external snippets. However, currently the code uses `null` as default lang for internal snippets, and only recognizes "java" and "properties" as lang values for external snippets.

The code should follow the specification (or else the specification should be updated). We may also want to update the spec to give a recommendation for adding a lang attribute for non-java inline snippets.