I had a professor once (it just so happens, also from Germany) who noticed me doing the exact thing that you're doing, but he didn't think that was a problem. What he thought was a problem was the apprehension I had over the pauses and the ums. That is what makes it sound unnatural, is your apprehension. He told me that if you need to pause for a moment, as a person who is familiar enough with the topic to teach it to others, the audience sure as shit needs a pause as well so that they can process the information you've just given. If you rattle it off faster than even you can think it through, it will go over everyone's heads and people will surely be bored to tears.
He said instead to embrace those pauses and let them help you get to the next thought. The audience will enjoy the presentation more and you will feel more comfortable as a result. All of my presentations after that have been much improved and I don't freeze up like I did when I started out.
Also of importance is practice. You simply have to just do this kind of thing repeatedly in order to get good at it. I hope this is of some help.