It’s in Portal. Ghost/ghost/portal at main · TryGhost/Ghost · GitHub
You could build a translated copy. It’s not too bad to get the portal code, edit, build, and host, but it does mean that you’ll need to merge improvements in Portal into your edited version.
There’s work ongoing to translate Portal, but I don’t think Portal currently works with {{t}} strings, based on this post: Hacktoberfest Project: i18n for Portal, Member emails, comments-ui & SodoSearch
Another option is to code your own member functionality, rather than using the Portal screens. See here: