Dutch Uncle was created in 1973.

Mijn oom is Nederlands.

The term 'Dutch' cousin or 'Dutch' uncle usually refer to someone that is treated as family, but has no actual relation to you. Typically a Dutch cousin would be the kids of some of your parent's best friends or perhaps step children of real aunts and uncles.

The original expression is a Dutch treat, in which you invite someone to join you for something but expect them to pay their own way. Possibly it dates from the many years the English were at war with the Dutch. There are other expressions in English that are uncomplimentary to the Dutch, such as Dutch uncle, meaning a stern and unforgiving task-master; Dutch-built, said of an ugly ship; and others too coarse for publication here.

Dutch-oom

French-oncle

German-Onkel

Greek-θείος

Italian-zio

Portuguese/spanish-tío

Russian-дядюшка