> “direct translation of _de perdidos al rio_, which is a saying meaning that once you have embarked onto a silly/dangerous/nonsense business you might just as well go through with it”
Essentially the same saying with the river metaphor exists in Finnish (ojasta allikkoon) but its implied meaning is the opposite of what you’re describing. There’s no suggestion of “might as well”, it’s just: “you were in trouble already but now you’ve really blown it.”
I suppose that fits the national stereotypes of adventurous Spaniards vs. cautious and pessimistic Finns.