Comment by akvadrako

si_LK is close to your requirements, but uses the comma for thousands. I don't think any locale uses underscore.

number: 12,345.67 date: 2025-10-14 09:45:17 paper: A4 (210×297)

https://lh.2xlibre.net/locale/si_LK/

I personally think en_DK is a better compromise.