I donít know if this is true, because, i have domain *.hr and content only in English and Croatian, and I am top ranked on google.de on targeted keywords I could send you address and keywords to see it!? It seams that only inbound link (link popularity) is the only way to rank site in top 10?!
Link popularity is certainly not the only way to the top of google, but it helps :)
I would guess that the keywords you are doing great for in google.de probably has a small number of competitors in German and hence your site shows up for a search. Are the keywords German or Croatian?