This thread started with a mention of Welsh macaronic songs. Here is a good example which has not been posted yet:
A ei di'r deryn du?

A ei di'r deryn du
To my dearest love
O cais fy nghangen gu
For I'm so deep in love.

Ni welaf yn un man
Such a damsel in my sight
A'r ferch mor lan o liw
She is a beauty bright

Mae'i gwallt yn felyn aur
Just like a ring of gold
A'i phryd fel eira gwyn
The truth it must be told.

Collected fromW. Sylvanus Jones of Llanllyfni , Sir Gaerfyrddin & published in 1941.