* Pass JapaneseUtil instance to StructuredContentGenerator
* Move body of createStructuredContent to an internal function
* Create _createStructuredContentGenericElement
* Wrap structured content in a <span>
* Change _createStructuredContent to _appendStructuredContent
* Add public appendStructuredContent function
* Add missing return
* Remove unused _createDocumentFragment
* Automatically assign lang=ja for content with Japanese characters
without an explicit language
* Add test
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