Not to mention, different builds per language for each OS. [1] This has never really made sense to me, but I'm sure there must be a good reason.
[1] - https://ftp.mozilla.org/pub/firefox/releases/68.1.0esr/linux...
[1] - https://ftp.mozilla.org/pub/firefox/releases/68.1.0esr/linux...
Historically reducing your download size by a couple megabytes has had measurable positive impact on conversion rates (from webpage visit -> install) so I expect this tradeoff made a lot of sense in the past even if it's less important now.
EDIT: One other thing worth note is that there's zh-CN and zh-TW, and I can imagine the Chinese government being very irritated about the idea of their release having separate Taiwan loc text/images baked in since according to them Taiwan doesn't exist. Major software like Windows has already had CN-specific releases for this (among other) reason(s).
The current practice is really a dimension-reduction, where a N×2 matrix [(CN, TW, HK, SG, MY...) × (simplified, traditional)] collapsed into a 2×1 vector (TW, CN).
The language labels should have been `zh-hans` and `zh-hant` if one means to differentiate the writing systems, and not the underlying linguistic variants.
According to Beijing government, Taiwan absolutely exists, it's just that it is not a sovereign state. Hong Kong has been handed over to China for more than twenty years and people in Hong Kong continue to use zh-HK – Beijing government seems to be OK with that.