Enable syncing of zh-tw and zh-hk for MediaWiki
/zh-hk is avaliable, according to phab:T51898, it's no longer possible to use /zh-tw
Right now, zh-tw and zh-hk are disabled in favour of zh-hant on Translatewiki.net. This is done presumably because the two are largely identical, so it's better to concentrate the translators' effort in one place. However, there are occasionally differences in usage between the two, leading to minor issues in user interface. (Furthermore, since the languages are disabled here, there appears to be no way to update previous translations bundled with MediaWiki.) I think a nice middle ground would be to keep the languages disabled in Special:Translate, but enable syncing for messages translated by manually editing MediaWiki:Foo/zh-tw or MediaWiki:Foo/zh-hk (if technically doable). This would allow tweaks to be made, while preventing translators from unknowingly duplicating effort. (This also applies to interface messages for Wikimedia wikis, if that goes through a separate mechanism.)
If there are previous translations in MediaWiki, they should probably be removed. I don't think we should enabled zh-cn and zh-tw. We've been using it like this for more than 8 years. I'll point Liangent here, a long time and knowledgable contributor to Chinese i18n.
Replying here on David's behalf because he can't get LQT to work for him:
There are real language differences between zh-TW and zh-HK in the MediaWiki files. For example, the word for 'bot' is '機器人' in Taiwan but '機械人' in Hong Kong. Note that both are written in Traditional Chinese (the Simplified Chinese equivalents would be 机器人 and 机械人).
Therefore using only one set of translations for Traditional Chinese would mean showing some wrong words in the Hong Kong interface (or the Taiwan one). I believe the same issue applies for Simplified Chinese: some the words used in zh-CN and zh-SG are different.
The variants could be labelled 'zh-Hant' and 'zh-Hant-HK' respectively, which would be BCP47-compliant. (Similarly, 'zh-Hans' and 'zh-Hans-SG' would be appropriate).
I actually discussed with Liangent before coming here (originally I had wanted to simply re-enable zh-tw and zh-hk fully, but they pointed out that this would completely mess up the "translated" statistics and cause maintenance issues). As Jdforrester quoted, there are some differences, and probably relatively more numerous of them in interface messages, since there are many neologisms in technical terminology. I think the question is whether the technical difficulties are a bigger issue than a UI that doesn't feel fully localised (to be clear, it's basically completely understandable, just imperfect).
Incidentally, while I don't use zh-hans / zh-cn / zh-sg, I would guess that this is a smaller issue on their side, as I think there is only a single Simplified Chinese translation for most software, unlike the case for Taiwan and Hong Kong.
Per phab:T51898, you're no longer possible to translate under /zh-tw, however, we still open /zh-hk, that's actually never disabled.