some questions to easy handling of Salome

Hello,Jo
1) is there an "undo-button"?

In current version "undo-redo" button is disabled
2) is there a log-file where any activity is recorded? My problem, for example: When merging nodes I don't get any message if and how many nodes have been merged. It is a bit cumbersome to be doing repeatedly "advanced mesh info" in order to control what happend.

You can  use File –Dump study to have file with Pythons commands to reproduce your actions.
Unfortunately there is no log file that you mean. Some information appears in Python console and in terminal.
3) is there a shorter way for deleting an entity than "Edit - delete". It seems that the delete-button does not do this.

No
4) Wen deleting all mesh-entities (meshs, submeshes, Hypothesis and so on) by doing repeatedly "Edit-delete", the most upper line of the mesh-entries in the Object Browser which is titled "Mesh" cannot be deleted. Does that mean that any mesh infos are still in the database, or do I have only geometric entities now? The latter is what I want.

You have only geometric entities
5) After creating a working plane (Geom module - New Entity - Basic) some 2D-commands seem to do their job in this working plane. Is it possible to have several working planes and to switch between them and how?

There is only one working plane for a moment and all related operations are performed in it. You can change working plane any time you need. You can create a few Local coordinate systems and use them to define working plane.
6) Scaling geometry
Geom-Modul - Operations - Transformation - Scale-Transform
Is scaling possible with respect to selected coordinate directions, for example only with respect to X, obtaining then a deformed  body?

No
7) Scaling a mesh. Is this possible and how? I didn't find how

No
8) is there an easy way to measure the distance between two locations?

In Geom you can obtain point coordinates “Measures - point coordinates” and distances between two objects “Measures –Dimensions-Mindistance” (eg 2 points)

In Mesh - node coordinates and lengths of edges

9) mouse-selecting entities for adding them to a group
I dont understand and dont succeed to select element with the mouse.
I make Mesh - Create group. Inside the Create-group-windows I select the Mesh, the Elements Type, I type in the groups name, I select "Standalone Group". But then it is not possible in the grafics window to highlight the elements and to transfer a selection by the add button. What am I doing wrong and how must I do?

You do everything right. It must work by clicking one element or a few with Shift button, with rectangular or using Filters to select elements in VTK viewer and then use Add button

Best regards,
Grigory