Initial overhaul at zh-CN YML translations (#2242)

This commit provides a heavy proofreading of zh-CN translations, and
mainly draws from the zh-HK source when a translation is found too
incoherent. Translating directly from en is used when zh-HK translation
is not available.

This commit also completes the tweet-toot (choo-choo, actually) for
zh-cn introduced in #2044. Some minor copyediting, as well as
additional en translations, has been fed back into zh-hk text.