A True-blue Bilingual

Surfing two languages is a dangerous sport, riddled with perils and pitfalls that affect even the best.
My daughter Laura, who makes her home in San Francisco, is one of the few true-blue bilingual people I know. 
Yesterday and while she was talking in Spanish to a monolinguist I overheard her saying: "... y dimos una fiesta de calentamiento de casa..." which I took as a quick, speedy translation of "a house-warming party." The problem here is cultural, not truly linguistic. People in Spain move into a new home and they do not throw a party to celebrate or warm it up. So, did the Spaniard understand what kind of a "fiesta" that was?
This is why I insist on the problem of culture and habits and customs... and not only on grammar and vocabulary.