What is the origin of names such as Wallachia, Wallonia and Wales?

All three names find their origins in the German language.

Wales: https://en.wikipedia.org/wiki/Wales

Wallachia: https://en.wikipedia.org/wiki/Wallachia

Wallonia: https://en.wikipedia.org/wiki/History_of_Wallonia

The English name "Wales" originates from the Germanic word Walh or Waelisc, which referred to foreigners who had been "Romanised". The same etymology applies to walnuts (meaning—nut of the Roman lands) as well as to the "wall" of Cornwall in Britain and to Wallonia in Belgium. Old Church Slavonic also borrowed the term from the Germanic, and it served as the origin of the names of the Romanian region of Wallachia.
https://en.wikipedia.org/wiki/Wales