something that immediately comes to my mind is that some "true Russian" names like Oleg(male)/Olga(female), Igor are Scandinavian and came from original Vikings that were asked to be warlords of Slav tribes. the division of Eastern Slavonic languages from Old Slavonic into Russian,
Belorusian and Ukrainian had happened a thousand years later so I think it is hard to distinguish the primary influence from secondary, tertiary etc. I am out of my depth in comparative linguistics but as a native speaker I can not say that there is more German than, for example, English words in modern Russian and Belorussian. I'll try to get some help from other forums where people are better qualified to discuss this.