I watched the extended cut of the movie and can’t seem to figure out why the director put that quote in there. Is this a hidden joke or a slip up that made it into the movie?
tl;dr: It's just a funny* line of PJ's.
* Your funny mileage may vary.
From an in-universe perspective, I agree with Himarm's answer.
From an out-of-universe point of view, we can add that the line, together with the whole dismemberment scene, comes from an idea of Peter Jackson's.
Quoting from the production and post-production comment track in The Two Towers - Special Extended Edition:
Mark Ordesky (executive producer): This is just a classic Peter Jackson kind of beat coming up. The idea that these Orks and Uruk-hai are arguing about the two little hobbits and they are just going to completely clobber each other any minute now ... it's a real throwback to Peter early movies
Later on, they talk about this specific line:
Rick Porras (co-producer): I was always curious if some of the more "Tolkien" fans like Christian Rivers had taken umbrage to the use of such a modern term as "menu"
Christian Rivers (previz supervisor): I'm not gonna query the script writers. My only issue is that for an Uruk-hai to say that, that means ... that suggests that he lives in a society where (a) he has a menu, and (b) that menu is selective enough to have food taken off it and put back on again
The orcs from the White Tower had just started a fight/finished killing the orcs from Mordor (Morgul) so they mean they are going to eat the dead orcs.
It’s funny because right before that, the orc that was just killed wants to eat the hobbits.
MAÚHUR I'm starving. We ain't 'ad nothin' but maggoty bread for three stinkin' days! SNAGA Yeah. Why can’t we have some meat? [His eyes rest on the hobbits.] What about them? They’re fresh. UGLÚK They are not for eating! GRISHNÁKH What about their legs? They don't need those. Ooh… They look tasty! UGLÚK [Shoves at the Orcs] Get back, scum! [The other Orcs are getting restless.] MORDOR ORC Carve them up! SNAGA [Moves towards the hobbits with his blade drawn] Just a mouth full. UGLÚK No! [Pippin and Merry recoil in fright. Uglúk jumps on the Orc and cuts off his head, which bounces off the hobbit's shoulders.] UGLÚK Looks like meat’s back on the menu, boys!! [The Uruk-hai and Orcs cheer and started tearing into the fresh meat, intestines flying, taking their eyes off the hobbits for a while.]
Uruk: "I'm starving. We ain't had nothing but maggoty bread for three stinking days!"
Snaga: "Yeah! Why can't we have some meat?"
[The orc rests his eyes on the hobbits.]
The quote was made because the orcs had not eaten any meat whatsoever for the past few days. So back on the menu would mean that the orcs had the opportunity to eat meat again.
It could just be as simple as the fact that they were tavelling and all they could carry in terms of food supplied was bread. Remeber they had just come back of a "failed" raid to try and capture Frodo and the ring and they had to move fast so meat was probably a luxury. I agree with the argument that the memu reference should have been excluded as argued above but in my opinion the idea that was created was they wanted proper food.
To be honest, I think that the fact that orcs have some idea/concept of a menu and even the fact that they can associate certain foods as being "put back on the menu" clearly suggests that there is some element of modern human culture that is known to the orcs. This can have multiple explanations. Either the orcs aren't actually as primitive as we would all think and in reality do in fact have some basic knowledge of dinning. Or, due to the war on men/elves and other advanced species, the tactics of "knowing thy enemy" has enabled the orcs to have some sort of concept of dinning due to their vast knowledge on the other races that has been gathered by gaining intelligence as it is known that the forces of Morgoth and Sauron did in fact have many spies and agents. This Intel could've ended up being engraved into their own orcish culture (as it is known that among orc society there are those who tend to food e.g. meats from local beasts that is prepared as meals).
However, it could just be that the orcs have just used the mainstream common tongue to exaggerate how they now have what they craved, meaty flesh. By exclaiming "Meats back on the menu!" clearly shows that the orcs are very excited about having their new favorite meal back as a choice, because it is rather obvious that the moldy bread they were given as rations is still an option for them to eat. These new choices of both meat or bread has created a sort of menu for the orcs, and with their knowledge of what a menu is, the uruk has exclaimed that meat is now finally back on their menu/back as a choice.