In case you don’t know it, the American phrase “Well, duhhhhhhh” is a sarcastic phrase that conveys the idea of “Well, isn’t that statement so obvious that it’s like being hit in the face with a board?”
The French equivalent would be quelle surprise.
What about other languages?
It’s the same in German, although the W is pronounced like a V and there is an umlaut over the U.
I’ve heard of a gesture that might fit. It’s French, I think. With the mouth open, put the tip of your thumbnail behind your upper front teeth. Pull the thumb forward until the nail slips off the teeth with a little click.
Roughly translated to English, it’s, “That’s so stupid it makes my teeth hurt!” French folk or other Europeans, feel free to comment or disagree.
For Spain the closest equivalent I can come up with right now is exxagerating ashtonishment. The same lines could be used to indicate true ashtonishment or that you find something ashtonishingly hilarious, it’s all in tone and gesture. So, not really any specific line.
¡No me lo puedo creer! (I can’t believe it), ¡Ostras, Pedrín! (from a pulp comic strip, Roberto Alcázar y Pedrín… think Batman and Robin without the fancy budget; Pedrín would say that when he was surprised), ¡no jodas! (you’re fucking kidding me), ¡venga ya! (c’mon…), en dos palabras, im presionante (two words: im pressive; quote from a particularly, ehrm, not too bright bullfighter)… etc. etc. etc. Some specific expressions translate across the Atlantic better than others, but the sarcasm travels just fine.
The statement itself is called a perogrullada or verdad de Pero Grullo, from the sentence verdades de Pero Grullo, que a la mano cerrada la llama puño: the wisdom of Pero Grullo, who a closed hand calls a fist.
In German, ‘ach nee!’ might work—‘ach’ is a sort of generalized interjection, denoting surprise, while ‘nee’, which is slang for ‘nein’, i.e. ‘no’, or some sort of negation anyway, makes the whole thing into a mock surprised rejection of what’s just been said—kind of like ‘you don’t say’, I’d say.
Japanese largely doesn’t have sarcasm. They’d just call you an idiot (baka) or let it slide.
Mandarin Chinese:
*Fei hua! * (“Wasted words” - in other words, “it was obvious, you wasted words needlessly”)
*
Ni **cai **ne? * (“Well, what do you THINK?”)
In Italian it’s “ovvio” or “ovviamente”–obvious or obviously. There may be a more colloquial way to say it.
By the way, my wife and her female friends, who grew up in the San Fernando Valley, say “obvi.”
In the north of England, you can say “Well, I’ll go to the foot of our stairs!”.
Or in the south you can say “You think?” with a raised eyebrow.
Another interesting question is what was used in American English before “duh” came into use. The Historical Dictionary of American Slang lists the first citation of “duh” in 1963, although it is a quote from Time magazine that claims “duh” is a “standard retort” among children at that time. Personally I don’t recall it being in wide use before the 1970s. I don’t remember any earlier phrase with the same connotation. Well, maybe “no sh*t, Sherlock”, a favorite of a high school friend of mine.
–Mark
An Italian speaking friend uses the phrase “Ma guarda!” to underscore those Homer Simpson exasperating or stultifying moments. The phrase translates to: “But look!” However long the 1st syllable of the 2nd word is drawn out conveys the depth of the event. 
“Nu, duh” seems to have been adopted by many Israelis. Another, slightly older phrase is “Bechayecha”, meaning “by your life”, with the Arabic version of the same word, “Bichyat”, being even more popular. For maximum sarcasm, elongate the first vowel: “Beeeeeeeechyat”.
In Mexican Spanish (different than Castilian Spanish in Spain) it would be “Pues, claro” ! which means, literally; “well, clearly” ! and used after something really obvious has been stated. 
OP example and a few others are either parodies of a dull witted comment or outright sarcasm; this example, and some others here, state the case, and the mockery or derision (if desired) must come from tone of voice, rolling of eyeballs, or whatever.
Definitely a FWIW.
In northern Mexico “A poco!” (sort of equivalent to No way!) is sometimes used as well. The first syllable of “poco” is generally elongated with a rising tone.
“You don’t say”
As soon as I read the OP, I messaged a Mexican-American friend with the question. He was born and raised in Guerrero, and upon emigrating to the US he made a concerted effort to learn to read, write, and speak proper English, and to be completely honest he spells better in English than most native-born speakers I know.
Anyway, I told him:
"Somebody has already answered, “In Mexican Spanish (different than Castilian Spanish in Spain) it would be “Pues, claro” ! which means, literally; “well, clearly” ! and used after something really obvious has been stated.”
And he said, “That could be correct, or we also use DUH.”
He also said, “We also use, : neta?”
“Neta?” is right on.
Not a direct equivalent, but I suppose in Danish you’d simply say “er det rigtigt!?” (“is that true!?”) with mock surprise.
In Swedish, probably “nähä, är det sant!?”, “är det sant!?” or simply “nähä!”, where “är det sant!?” means the exact same thing as the Danish phrase above, and “nähä!” is a kind of surprised negation (“no way!”).
In Swedish, there’s also “jaså det menar du inte?” (literally “oh you don’t mean that?”, but closer in meaning to “oh you don’t say?”).
“Well, duh” is an interjection. Romanians use an interjection too, which they can prolong in the same way Americans pronounce “duh.”
The main difference resides in the fact that “well, duh” often expresses sarcasm whereas the Romanian one is characterized by jovial humor.