I was searching for this the whole day, but i found a solution
There is a problem with translating the string:
MYSITE is your new site. Login as USER using your existing password.
And When you have another translation file, translating Wordpress. Because there is a double space between "...%2$s" and "is your new site..." on line 226 in ust-wp-signup.php. When you remove the unnecessary space, the string translation works! Would you like to update this in an upcoming release?
Other users might find this useful.