splitting zh_HK

[wpi3]

@OpenTTD/zh_tw

@frosch123 This has been stale for more than two years now (starting from OpenTTD/OpenTTD#9801). When can we get the split done?

I've been maintaining translations for zh_TW for quite some time now and frankly speaking the team feels pretty dead apart from me. We might as well just change zh_TW to use Hong Kong translations, which would save me decent amounts of effort to lookup the Taiwan terminologies unfamiliar to me 😃.