Isn't this intentional? See https://ne.wikipedia.org/wiki/विकिपीडिया:अंक_परिवर्तक - when setting the dropdown to १२३ it is displayed correctly.
If that does not work for you, please provide a link where the problem can be seen.

Aklapper,

Would it be possible to make Devanagari Numerals by default(because it's
official) and then let people change it to whatever system needed

​Please see here :
https://ne.wikipedia.org/wiki/%E0%A4%B5%E0%A4%BF%E0%A4%95%E0%A4%BF%E0%A4%AA%E0%A5%80%E0%A4%A1%E0%A4%BF%E0%A4%AF%E0%A4%BE:%E0%A4%A1%E0%A5%87%E0%A4%9F%E0%A4%BE%E0%A4%A5%E0%A4%A8

TOC has devanagari numerals and ol has English numerals what an epic
inconsistency ?​

I can find the community consensus if needed to convince you .

By default the option in that (custom and on-wiki) dropdown is "डिफल्ट" and I assume that default is defined somewhere in https://ne.wikipedia.org/wiki/मीडियाविकि:Gadget-Numeral_converter.js ? (I don't know, I'm not a developer.)

The authors of that script might know? https://ne.wikipedia.org/w/index.php?title=%E0%A4%AE%E0%A5%80%E0%A4%A1%E0%A4%BF%E0%A4%AF%E0%A4%BE%E0%A4%B5%E0%A4%BF%E0%A4%95%E0%A4%BF:Gadget-Numeral_converter.js&action=history

Might be something to discuss on-wiki and off-topic here in Phabricator as this seems to be on-wiki code. But if the default is somewhere defined in the codebase of MediaWiki itself it might be totally correct here in Phabricator.
I've asked in #mediawiki-i18n on IRC if someone knows...

List item numbering (<ol><li>) cannot be customised in the same way as other numbers are. It seems only Firefox supports Devanagari numerals: https://developer.mozilla.org/en-US/docs/Web/CSS/list-style-type

Nepali could be included in the list of languages added in T15896, would that be okay?

Hi Niklas,

Sounds great !

Thanks,
Saroj

There are no clear instructions where and how this should get fixed, hence removing the good first task tag.