Mandarin - add a pinyin toggle button#41

Add a pinyin toggle button for Mandarin.

Thank you for this suggestion, Simon - we really appreciate it! Are you thinking in particular about having this toggle for the video transcripts?

Yes, in particular for the video transcripts, but also in other areas of the site the option of toggling pinyin on/off would really benefit our learners.

Thank you very much for the extra info!