My educated guess is that the localization data for every language is pretty massive, so per-region downloads can include the text for their region without dragging along a few extra megabytes of text/images for other regions. If you swap regions after installing it can probably download on-demand.

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).

`zh-CN` and `zh-TW` are generally just old-school ways to say "simplified Chinese" and "Traditional Chinese" (two similar writing systems). The confusion comes in as the Chinese language family has multiple dialects and variants, and there are multiple writing systems.

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.

zh-CN and zh-TW has nothing to do with that, and I'm virtually certain there are no "Taiwan images". in applications which provide zh-CN and zh-TW, including Firefox, the former means Simplified Chinese and the latter means Traditional Chinese. theoretically, it could mean Taiwanese Chinese, but then there would also be zh-HK, zh-MO, zh-SG...