Here's a few more:
Sorry if you guys have these already, I haven't read all 20 pages of this discussion!
There's a famous play on words in French:
Je pleure dans mon coeur comme il pleut sur la ville.
Literally, "I cry in my heart as it rains on the town." The verbs for "to rain" and "to cry" are very similar in French. It's a very sad image, but kind of neat as a wordplay.
Chinese characters are also pretty cool sometimes. I don't know if you'd call it a true idiom, but you can frequently add characters or radicals (little word-bits that have some meaning but can't stand on their own) together to get totally different things. For example:
"bright" = sun + moon
"good" = boy + girl (My teacher said, "In Chinese, one girl and one boy means perfect" because that's considered the perfect family.
"fine silk" = tiny + small
"read a book" = watch + book
"America" = beautiful + country
"China" = center + country (China was always in the middle of old Chinese maps -- and why shouldn't it be?
"television" = electricity + vision
A true Mandarin idiom is, "Ma ma hou hou" which is literally "horse horse tiger tiger" and can mean "okay" "sort of" or "a little bit." So if someone asks how you're doing, and you're doing neither bad nor good, you could say, "ma ma hou hou." Or you could use it to mean "sort of" or "a little bit" if someone asks, "Do you speak Chinese?" and you want to say something along the lines of "Yes, but I've only been studying it for a little while, so I only speak a little bit" but your Chinese isn't yet good enough to say all that!
Also, "da xuesheng" literally means "big student" but it actually means "college student." "Da" is also used in front of the word for school to make it mean college, but in other contexts it just means that the object it describes is large.
If a Vietnamese girl was shopping at the mall and tried on an outfit that turned out to be a LOT more see-through or revealing in the dressing room than it was on the hanger, she might explain the problem to the friend she's shopping with by using an idiom that literally means, "All my treasures have been brought out into the broad shiney daylight day!"
In French, an all-nighter is "une nuit blanche" (literally, a white night).
Also, if you take the French word for street lamp and change the ending so as to "turn it into" a female person, instead of meaning a female street lamp, it means a prostitute. Though I don't think this particular term is used much anymore, you tend to see it in old novels, back when the street lamps were actual lamps before lightbulbs were invented. Why???? you might ask. Well, think about it... it's 1900 in Paris on a foggy night, you're walking down the street and you see a beautiful woman standing in the pool of light under a street lamp... what are the odds that any normal person has nothing better to do all evening than stand around under a street lamp?