I think the most idiomatic English terms to criticize someone who is incautious or too eager to take big risks spontaneously would probably be "reckless" or "ra

I think the most idiomatic English terms to criticize someone who is incautious or too eager to take big risks spontaneously would probably be "reckless" or "rash". :-) (In English, "imprudent" refers more often to actions or decisions rather than to people.)