We need to consider Japanese and a few other locales that don't use 3/6/9/12 as the breaking points.

Here's what ChatGPT told me:

Where Factor of 10^3 Doesn’t Work
Here’s a summary of locales where the standard K (10^3) and M (10^6) approach doesn’t apply directly:

• East Asia (Japan, China, Korea): Uses 10^4 (万) and 10^8 (亿, 億, 억).
• India: Uses 10^5 (lakh) and 10^7 (crore).
• Older European Long Scale Countries: Countries using milliard for billion (though less common today).

I think it would be good to get a large list of locales, loop over them, and then format 9, 99, 999, 9999, and so on, using our hacky polyfilled version. Then find where it doesn't look correct (i.e. where it causes 0 to show up) and fix those.