The option to manually set the labels in the login form should be available, but unfortunately isn't due to a bug. I have logged this issue, and it is pending to be fixed in one of the next internal builds.
However, it is possible to localize the labels used in the login form to any language through Sitefinity's "Interface Labels & Messages" feature (under the Administration menu). You may add languages to the frontend ("public content") and/or backend settings, then add translation to the labels that take part in the login form.
1. On the main menu go to Administration -> Settings
2. Under Basic Settings, click "Languages".
3. Depending on the destination of your custom login page (frontend / backend) add German as a language, and Save the changes.
4. On the main menu go to Administration -> Interface Labels & Messages.
5. Locate the following lables, and edit their German translation (for easy locating, select "Labels" under "Filter by type" on the right filter bar):
For the UserNameLabelText: Username (Used in : Labels , Resource key : Username)
For the PasswordLabelText: Password (Used in : Labels , Resource key : Password)
For the LoginButtonText: Login (Used in : Labels , Resource key : LoginCaps)
Once those are translated and saved, restart your server, and the login form should be localized whenever German language is used.
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