This problem is not limited to multi-site installations but also on simple WP installations. And not only does /wp-login.php resolve to the “hidden” backend but also /wp-admin. So in effect is this security feature DISABLED and USELESS. This should be fixed a.s.a.p.
I translated the default.po file with https://localise.biz/free/poeditor, downloaded .mo and .po files and named them exactly as you did “contesthopper-de_DE.po” and contesthopper-de_DE.mo”. Uploaded into the language directory and – voilà – works. Maybe you forgot the .mo file? Or maybe it didn’t work because you changed the default files?
Unfortunately not all translations have taken effect. Some lines I DID translate but they still show up in English. Any ideas how to remedy this?
