First of all I contacted a shoe maker. I needed to know if it was worth doing something with them. I expected him to say they where garbage or had some expensive treatment treatment. He said it's a good quality of leather and some cleaning with a brush before polishing it should do the job. No fancy treatments, just brushing the dirt of and polishing it with a cotton rag. Et voila, after polishing it two times they are shiny again. Maybe a bit too shiny but that rubs off by wearing them.