Hi,
Not sure if this is being done using custom policies or user flows however this guide explains the setup for localisation:
https://learn.microsoft.com/en-us/azure/active-directory-b2c/localization
Additionally this is the list of localisation IDs:
https://learn.microsoft.com/en-us/azure/active-directory-b2c/localization-string-ids
I have found that the learn list is not extensive so you can also head to user flows -> click on any flow -> Languages -> Enable customisation -> click on a language. This will show a panel with all the pages with a link to download the default customisation file.
It is also worth noting that some of the localisation IDs are spread across multiple 'screens' and therefore have different files. I believe the screen you are looking at is under 'Local Account Signup Page'. If this is the unified page then I would expect the string IDs to be the same and they could just be missing from the default file for this page.
I hope this helps.