Thou shalt love the country in which thou wast born.
I would have expected “Du sollst das Land lieben” and also “… in welchem du geboren bist”, but this looks to be from a very old source text, so I imagine it would sound as odd in modern German as “thou shalt” would sound in modern English.
You are absolutely right.
It sounds archaic. Maybe it is a line from an old song or poetry.
My first idea was an old bible translation, so your comparison to “thou shalt” is fitting.
However, I couldn’t find the source.
She doesn’t say 〝at first〟.
You are absolutely right in this.
The given sentence is something someone living in the middle ages between the years 1100 and 1300 would have said. It is not at all how Germans speak nowadays, same as no English speaker today talks like Shakespeare anymore.
The sentence is taken from a book published in 1884 about knights, in which a historian wrote about what he read about the mentioned time period. (The sentence was not added by me, I just did a Google search to answer your question.)