Italian: b6 1 #4
French: b6 1 2 #4
German: b6 1 b3 #4
His words translated: b6 = flat 6, b3 = flat 3, and #4 = sharp 4, the numbers referring to scale steps.
In C major, these chords would be:
Italian: A-flat - C - F-sharp
French: A-flat - C - D - F-sharp
German: A-flat - C - E-flat - F-sharp
Through an enharmonic change, the German augmented sixth chord becomes the dominant to the Neapolitan:
A-flat - C - E-flat - G-flat resolving to D-flat major
Otherwise, they tend to strongly resolve towards I 6/4 chords.
Walter Ramsey