Mandarin Chinese is a notoriously difficult language to learn - a labyrinth of semantic tones, elaborate characters and obscure idiomatic phrases. And in China, a land of infinite linguistic diversity, the government has spent decades struggling to unify the country under that one language, not without some controversy. David Moser, the author of “A Billion Voices: China’s Search for a Common Language,” who has lived in China for more than two decades, talks about the thorny politics of Mandarin Chinese, which is known in Chinese as Putonghua. How hard is it to learn Chinese, really? There are aspects of Chinese that make it hard for foreigners to learn, and there are aspects that make it difficult