I would like to make a bilingual book, with the (original) German text on right pages, and the English translation on the opposite left pages.

I am looking for ways to do that.

One idea is to consider the German text as the main text (it is longer than the English version), and let it flow over the right pages only. The English text is then a kind of illustration, that is inserted manually as a text box on the corresponding left pages. I don´t see exactly how to implement this, and it is not very flexible of course.

Any help would be very much appreciated!

It is helpful to think of one text as the main text, ideally the longer one, so the translation can be shorter.

Perhaps you could use page float next for this, but it helps if paragraphs of the main text are short and cannot be split across pages.

Would it be possible to span a table over a spread?

Unfortunately not, tables cannot span pages horizontally.

Ok, This is my next suggestion (without knowing if this is actually possible, @mikeday will probably tell us):

We would need two streams (these are called stories in InDesign), one for the even pages and one for the odd pages; the odd pages will be named pages under a different name, so each stream will have its own flow.
We need related identifiers for each source lang <p> and the trans lang <p>, e.g. <p id="sl-1"> and <p id="tl-1">
Now a javascript will need to tell us what the vertical position of each new <p> is. We calculate the height of the previous p, and create css to do a padding-bottom of the shortest so its length will match the longest. Now rerun the whole thing and the paras should align. I cannot write this (yet) myself but I envision this could be a viable way. good luck.