No need to apologize! Your English is far better than my German
As @petri says, the built in plone.app.discussion in Plone lets you set up a forum. Looking around I don't see any good user docs. The package has some more technical info:
Generally what you do is (assuming Plone 5):
- go to Site Setup
- go to Discussion control panel
- check the box "Globally enable comments"
- there are many more settings on that control panel
- press Save button at the bottom
- go back to Site Setup
- go to Content Settings control panel
- select (for example) the Page content type
- check the box "Allow comments"
- press Save button at the bottom
Now go to a page on your site, e.g. the front page, and at the bottom of each page you will see a set of commenting fields.
Enter a comment (by default they're just plain text but you can change that in the Discussion control panel; email notification and comment moderation are other settings), press Comment button, and then you'll see it's possible to have threaded comments.
Probably the best way to use this feature is not to enable comments on Pages (that was just an example) since every page in your site could be commented on.
Instead create a custom Dexterity content type and enable comments on just that content type
- go to Site Setup, click on the Dexterity Content Types control panel
- clone the Page content type and call it a "Forum Post"
- go back to Site Setup -> Content Settings control panel
- select Page, uncheck the box "Allow comments", press Save
- select Forum Post, ensure the box "Allow comments" is checked, check also the box "Can be used as a default page", press Save.
- go back to the front page of your site
- add a folder called "Forum"
- when looking at the new Forum folder, click the Sharing toolbar button, check the box "Can add" for "Logged-in users", press Save (this means all your site's users, if they're logged in, can add a Forum Post in this folder)
- click the "Add new..." toolbar button, click Restrictions, select "Select manually", uncheck everything except Forum Post, press Save (this means only Forum Posts can be added to the Forum folder)
- click again the "Add new..." toolbar button, and add a Forum Post
- At the bottom of this (and all Forum Posts) your site users can comment and reply to comments. E.g. http://obscure-mesa-94101.herokuapp.com/Plone/forum/first-post/view
You'll have to configure maybe a different default workflow for Forum Posts so that they're always visible to your logged in site users, or you could activate (in Add-ons control panel) the Placeful Workflow add-on, and then set a policy on the Forum folder so it uses the intranet workflow. More fun.
But definitely doable.
You're free to play with this site, http://obscure-mesa-94101.herokuapp.com/Plone (log in with admin/admin). It is a free Heroku deployed Plone 5 site, which you can create on your own from here: Essayer Plone — Site