In German, numbers are spoken from left to right, as in English, but ones and tens are always swapped. So 23 becomes three, twenty, and 135 becomes one hundred, five, and thirty:

• Achter Stock (8)
• Achtzehnter Stock (18)
• Achtundzwanzigster Stock (28)
• Dreizehnter Stock (13)
• Dreiundzwanzigster Stock (23)
• Dritter Stock (3)
• Einundzwanzigster Stock (21)
• Elfter Stock (11)
• Erdgeschoss (0)
• Erster Stock (1)
• ...

Oh, and the first floor is called the ground floor and is not counted.