See Germanic.The tribes that came west also split up and developed Proto-Germanic into the languages German, Dutch, and Frisian....
The Germanic languages are widely presumed to have descended from the so-called Proto-Germanic language.
Repeating the exercise, we analyze the related languages - Proto-Germanic, the ancestor of Latin, Celtic, Greek, Baltic, Slavic, Albanian, Armenian, Anatolian, Hittite, Indo-Iranian, and Tocharian.