To the Caucasians, wearing shoes inside the apartment is part of their culture. I am prepared for litigation at the end of tenancy. If the Landlord-Tenant relationship was good, the German tenant would have responded positively. In my observation, the marble flooring will fade after 2 years of tenancy and may need another polish to maintain it. Caucasians wear shoes into their apartments as part of their culture.
Japanese tenants do wear slippers inside the house.
Singaporeans of various races keep their footwear outside the house. Get to know your new tenant. Don't hide behind the past. Live for the future and you can be happy as you want to be. Be glad that your unit has a tenant in this massive glut of investment condos.