Please provide tips for designing the ground floor plan

Is there no universally valid definition for the term longitudinal orientation

No.

What if there is no specification in the text part?

There isn't one. But in the legend to the graphic part it clearly states main ridge direction. However, the development plan is not yet legally binding, so perhaps clarifying formulations will be added in the final version. If not, I would say: in dubio pro reo. Since no roof shape is prescribed, a square floor plan with a hipped roof should be possible and likewise this:

If the latter were the case, then a gable roof with an east-west ridge direction might be possible, even if the house has smaller dimensions in the east-west direction than in the north-south direction....

A picture is worth a thousand words:

Is there no universally valid definition for the term longitudinal orientation if no definition is given in the text section?

As far as I know, there is no standard definition for this, which is why I would consider the regulation to be indefinite. Often, when a house axis is specified, the text section of the development plan includes the explanation that the municipality envisions a format ratio of the floor plan of 5:4 as the minimum inequality between house depth and width. It is also quite common in practice to insert a roof ridge of a symbolic half meter length into hipped roofs on square floor plans (which would actually be tent roofs), which requires lateral roof slopes that are approximately 2° steeper.

Unfortunately, there is nothing about that in the text section.

There is unfortunately nothing about it in the text part.

I wouldn't even call it "unfortunately." What is not specifically defined gives you freedoms. Thus, a wide house from the street view with less depth, as you wish, could also be possible, giving you a larger garden area. Only the roof ridge must run in the east-west direction.

Of course, it can happen that the guy at the building authority complains because that was not originally intended by the official planners. But if it is not explicitly prohibited, it should be allowed.

Since I am not a construction law expert, the above is of course not a legal assessment but a personal opinion. If you want to commission a local architect with the implementation of your ideas, you could already consult them about this in advance.