"Do you have the same problem when using other Windows browsers?"
Yes. I have the same problem using IE, or Chrome, the same as when I use my preferred Firefox browser when accessing the Numbers Spreadsheet from my desktop.
"Are you able to type Nepali in all the other icloud iwork apps?"
Yes. I am able to type in Devanagari script using the icloud e-mail app via the Firefox browser on my Windows 10 Desktop PC.
Contacts - Yes
Calendars - Yes
Notes - Yes
Numbers - Yes and No
As a test, I tried to create a NEW spreadsheet in Numbers.
When I created the spreadsheet it asked which language I would like to create it in.
I don't remember answering a question like that when originally creating the file on my iPhone.
However, the file on my iPhone allows both English and Devanagari text to be entered.
On my iPhone, I have English, Hindi, and iTranslate keyboards loaded. I use all three keyboards in the file.
Neither Nepali or Hindi is a choice in the list of available languages when creating the spreadsheet on my Windows PC.
Here's the curious part...
The spreadsheet would NOT let me enter Devanagari text into the body of the spreadsheet.
When I switched to the Nepali keyboard on my desktop PC, and tried to enter Devanagari Text, nothing happened as I hit the keys of the keyboard.
When I switched back to the English keyboard on my desktop PC, pressing the keys on the keyboard worked again.
I tried switching back and forth with the same results.
I just happened to leave the keyboard in the Nepali mode, as I was trying to exit Numbers.
I thought I better name the file before leaving numbers, and to my amazement the file name was in Devanagari!!!
So the test was kind of a mixed bag.
I have also tried to type Devanagari into this reply.
The Devanagari keyboard does enter the Nepali text into my response, but fails when I try to "post" my response.
The error reads, " ! You have included content in your post that is not permitted. "
Once I remove the Devanagari text, the reply can be "post"ed successfully.