"There has not been a 'non-literate' Britain for a very, very long time. "
Not necessarily true - the ability to read doesn't mean that it was used for everyday life
In Ireland it was further complicated by the two languages - singers and storytellers mistrusted the printed texts and only used them to supplement oral ones - we have several examples of that from both singers singers and storytellers
I once checked the rural literacy percentages and was staggered to find how many people couldn't read past the middle of the 19th century in England
The towns were higher but even the use of literacy skills was patchy and confined largely to the wealthier classes, even into the 20th century
My grandfather, as a merchant seaman, was part of the the setting up of the Maritima Workers Education Association for seamen
Literacy became a political weapon in the 1929s depression - being able to read was reckoned to be a way to fight exploitation
None of this is definitive of course but one of the factors of our folk songs is that they were found in the countryside rather than in the towns

I belive the 'German' references owe more to general Xenophobia mixed in with the mistrust of strange men entering your homes at will
There are pop songs of the early twentieth century of Spaniards who "blighted my life" or italians like "Oh, Oh Antonio" who seduced wives or girl-friends , given the opportunity

"PS Aren't we all playing nicely?"
So far - early days yet :)
Jim