Many thanks, JBW, this worked. Honestly, never in my life I would have found that.
So for others with the same problem:
- In the backend go to international - translations
- in the first block "changing translations" choose part: "template translations" and your template and language, then click on the button to the right below.
- then search for the word you want to change
A problem though, if it is a word for which several results are found, it only shows one of them and I don't see how to get to the others. But in my case, there is only one result "startseite", I simply but in "Shop" instead, save and it's changed in frontend.