I see your question nevertheless, so here is an answer.

When I chose the orientation of the text on the side of my first book, I did not give it much thought but simply chose what seemed consistent for me: usually, one takes a book looking at its front and when one then looks at the side, text going upwards is easily readable. I have never known of any German, English or whatever standards for this. Among my countless bought books, every orientation occurs, so I was not aware of any system for it. Now that you mention country-wise conventions, I am becoming aware that there is such a thing. Maybe. I do not know how well such conventions are followed. Anyway, German publisher suggests German orientation. Frankly, I do not care, except for one thing: it should be consistent on all my published books so that they can be browsed easily. The large, clear font helps.

For a change, look at The Trouble with Physics (black cover editions)!