Usually "entweder - oder"-questions are answered by naming one of the options, e.g. "Möchtest du Ketchup oder Mayo(naise) zu deinen Pommes?" - "Ich möchte (bitte) Ketchup."
Today two friends of mine claimed that answering "Ja" would by equal to choosing the first option. This is the first time ever I heard that claim and I am very skeptical. Hence I would like to know: Are they correct? Could I answer "Ja" when asked to choose, and "automatically" opt for #1?
Disclaimer: We're all native german speakers, but grew up in different regions. I stem from north of Hamburg, while my friends grew up close to Berlin. Maybe this is a dialect-"thing"?