The motto of the Prince of Wales is "Ich dien", "I serve" in German. Why in German?

There are no exact historical accounts as to why but according to popular ones it is said that one of the earliest Princes of Wales, Edward "The Black Prince", obtained the motto from the badge of John I of Bohemia following the battle of Crécy. Another contributing factor usually claimed is also the fact that "Ich Dien" bears similarity to the Welsh phrase "Eich Dyn" which means "Your Man".

https://en.wikipedia.org/wiki/Prince_of_Wales%27s_feathers#Popular_accounts_of_origins

The similarity of Ich dien and eich dyn begins when King Edward promised the Welch a King who could not speak a word of English. Then introduced his newborn son and mispronounced the words.
http://www.infoplease.com/dictionary/brewers/ich-dien.html