See attached patch.  It adds the German T1 keyboard layout to Emacs.
Before submitting it to <bug-gnu-emacs@gnu.org>, I decided to discuss it
here, following the instruction on “Sending Patches for GNU Emacs”.

There are some limitation due to Quail operating on top of an ANSI
keyboard, which has one key less than an ISO keyboard, and which has no
AltGr modifier.

There is however a limitation that seems unnecessary.  The ° and ^ keys
behave differently:

  * The typed characters initially appear with an underline.

  * When doing <C-5 °>, then one has to press another key to get: °°°°°

  * When doing <C-h k °>, I have to press return for `*Help*` to show
    up:

    “° (translated from <return> °) runs the command self-insert-command
    (found in global-map), which is an interactive built-in function in
    ‘C source code’.”

I don’t know if that can be fixed, and how.  Maybe I could step through
the code to see what’s going on?  But debugging `self-insert-command`
sounds painful.

Another limitation: When entering `-` with `typo-mode` enabled, it does
call `typo-cycle-dashes`, but the dashes don’t cycle.  This is genuinely
annoying.  Again I would like to but don’t know how to debug that.

Still, even with these limitations, I believe that the `german-t1`
keyboard layout is a useful addition.  I used it all the time for the
past months.  In fact I have configured my Emacs to automatically switch
the keyboard layout to `german-t1` when a file is loaded with
`ispell-local-dictionary` set to a German dictionary.