Answering the question was harder than I thought. Through a friend whose daughter studies ancient languages and is interested in celtic, I have now received an answer. She herself didn't know either, but the question was forwarded through her to a specialist at an English university.
The words are a mixture of Anglo-Saxon, Norman and French. They should actually come from the period around 1390 to 1410.
In German the saying means:
(Klar) und sicher
(dessen) Huld (ich) dort erwarte in Hoffnung
Whereby even the specialist is not 100% sure about the first part.
My English is too bad to translate this. I will try it anyway:
(Clear) and safe
(whose) mercy (I) expect there in hope
I have now still asked to decipher the individual letters for me. Otherwise the flag can not be labeled.
I hope that I will also get this answer.