"The Tau are the only faction in the 40k setting that actively pursue diplomatic channels with other factions." Here are some transcripts detailing some of the more typical negotiations. For extra lulz, imagine the Water Caste representative as having the voice of Starfire from Teen Titans:
Bad Moon: Oi, givez us yer shooty flash or we'll crump the lotta ya!
Water Caste: Are you sure we can't interest you in these grain harvesters?
Bad Moon: Orks don't eat none a' that, we eatz squigs! Now givez uz the flash!
Water Caste: O-okay.
Bad Moon: WAAAAGGHH!
Evil Sun: Hey, we 'erd youz lot gave plasma shootas to boyz. Giv us sum a dem.
Water Caste: Well, I'm sorry, but we don't seem to have any left.
Evil Sun: WAAAAAAGH!
Water Caste: We would like to be the first to welcome you extragalactic travelers to our stars—OH SWEET F-!
(Or, if you believe in 6th Edition Meta...)
Water Caste: Oh dear! This is absolutely dreadful! I require the nearest deployment of battlesuits! And bring the Riptide as well!
(One battle, three dead Zoanthropes, a dozen dead Warriors, a Flyrant and a Swarmlord later...)
Fire Caste Commander: Ah, the power of the Greater Good and intergalactic diplomacy triumphs once again! *Grinds hoof into the Swarmlord's face. Repeatedly.* It is delicious!
Water Caste: The Tau Empire is pleased to open negotiations with those that claim to be the most ancient enemies of the Chaos Barbarians.
Trazyn the Infinite: My dear blue lady! Do you realize you are the spitting image of O'Shaa'mal, the Water Caste envoy who first made contact with the Imperium of Man and precipitated the Damocles Gulf Campaign? Now strike a dignified pose while I prepare a stasis grenade.
Nemesor Zahndrekh: *riiip* These clothes are terrible! In my day we had reliable stitching. *riiip* Terrible! This merchandise is a shame to Necrontyr weavers everywhere.
Water Caste: Silly robot, those clothes are for Tau.
Vargard Obyron: *Stab* You've said too much.
Water Caste #2: I hear that random strips of cloth are in fashion these days. I'll even give you a discount on those you're looking at now.
Zahndrekh: Really? Sold!
Water Caste: How would you like to pay for these spices and garments?
Noise Marine: I would like to offer a cultural exchange.
Water Caste: What type of cultural exchange?
Noise Marine: A little something I like to call *puts sunglasses on* Rock n' Roll.
Doom Siren: YEEEEAAAAAHHHH!
Water Caste: We would like to extend the hand of friendship to—
Farseer: Fuck off, newfag!
Water Caste: But we could help if—
Farseer: I’d rather die!
Water Caste: We would like to extend the hand of friendship to—
Farseer:I predict that an alliance will lead to the ruin of a dozen Tau Septs while ensuring I get a shiny penny next month We already agreed to accept your hand of friendship. Honor our terms to go to war.
Water Caste: How many should we commit?
Farseer: Bring your Mon'Keigh allies, they will serve as excellent soldiers against the forces of Chaos we are about to fight and lure them to your worlds so we can plan an ambush together.
Water Caste: It is good to see an elder race taking responsibility and welcoming its juniors into the galactic community.
Archon: Yyeeeeess...younger races are so soft and supple...they require guidance in this harsh universe, for they often cannot comprehend the ways of their betters.
Water Caste: Sir, why have you wedged your armor's codpiece between my buttocks?
Archon: What gave you the impression that I was wearing a codpiece?
Water Caste: We would be glad to discuss terms of an alliance with an elder race.
Archon: Excellent. marshal your armies alongside mine and we will crush the enemy. As for negotiations, I invite you and your people to visit us in Commorragh. My warriors will show you the way.
Water Caste: You most definitely have a deal! How many can you accommodate? We wish not to be a burden.
Archon: Bring as many of your people as you want...Commorragh has room for all. A word of advice, dress lightly. Easy access
Water Caste: This vessel, in service to the Tau'va, hails the unkno-
Water Caste: On behalf of the Tau Empire, and in the name of the Greater Good which we serve, we are delighted to welcome you, our newest Gue'vesa—why do you have your hands on my breastplate? What are you doing with the restraints of my brassiere? Please stop that.
Human soldier: Hey, I didn't surrender, betray my entire species, and risk the wrath of the Inquisition just because you bluies are decent talkers.
Water Caste: *sighs* Okay, good point...for the Greater Good...I guess.
Human soldier: Whatever gets you through the next 30
hours seconds sure.
Water Caste: As a representative of the Tau Empire, I wish to offer you—
Angry Marine: FUCK YOU, WEEABOO SPACE COMMUNIST!
Water Caste: Actually, your human communism is in no way comparable to our—*PAWNCH*
Angry Marine: ALWAYS ANGRY!! ALL THE TIME!!
Water Caste: I'm glad we could make a trade agreement with your chapter, captain.
Blood Raven: Likewise, until we meet again.
Water Caste: WHAT DO YOU MEAN THEY TOOK THE SHIP?
Water Caste:We are honored to have the commander of Bloo-
Indrick Boreale:Tehre is noh time to beh lohst!
Indrick Boreale:Batel Brothas!
Indrick Boreale:Spehss Mahrens, todeh the enemeh is at oua doar! We know oua duteh and we will do eet. We fight for our honor as Blod Rehvens,
Water Caste:*Raises finger to interrupt* Si-
Indrick Boreale:as SPESS MAHRENS, and we fight in the nehme of the Empra!
Water Caste:Very good! Now back to matter at-
Indrick Boreale:And if we die this deh we die in gloareh, we die heroes' deffs, but we shall not die, no! It is the enemeh who will tehste deff and defeat!
Indrick Boreale:As you know! Moast of oua battle brothars are shtehtioned in SPEHSS, Pruhpeared to deep strike! Oua perimeter has been pruhpeared in the even dat oua enehmies should be so bald and so foolish. We have plehced numerous beacons, allowing for muhltiple, simuln-tehneous and devashtehting defensive deep strikes
Indrick Boreale:The Codecks astartees nehmes this maneuvah Steel Rehn. We will descend upon the foe, we will ovawhelm them - we will leave none alive! Meanwhile oua ground fawses will ensue the full defense of oua headkwaters
Water Caste:A-ha.. Hm. We do-
Indrick Boreale:We are the spehss mahrens! WE ARE THE EMPRA'S FUREH!
Water Caste:*Visibly confused*
Space Marine: Status report: the xenos have reached our positions! Let the light of the Emperor fill your hearts, brothers! Send these beasts back to the slag-hole they have—one moment…damnation! Another foul xenos incoming! DESTROY-
Water Caste: WAAAAAIT, mighty Gue'ron'sha! I come with offerings of cooperation and understanding! I wish you no harm! For now, anyway…
Space Marine: The Codex Astartes says, "Suffer not the alien to live." Soooo—*BLAM* Xenos scum.
Space Marine: WE'RE BUSY, ALIEN! YOU WANT COOPERATION, GET YOUR FORCES OVER HERE AND TELL THEM TO START SHOOTING THOSE OTHER SCUM-SUCKERS NOW!!
Water Caste: O-okay! Fire Caste, I request a deployment to my position to engage the enemy. Not the Gue'la warriors, the other ones!
Flesh Tearer: SHUT UP YOU FUCKING SALMON! SUSHI TIME!
Water Caste: I am sorry, I do not know—what are you doing to my leg?! AAHH! AAAHHH!! AAAAAAARRRRRRGGHH!!!
Flesh Tearer: OMNOMNOMNOMNOM…
Water Caste: Surprising times are when a notorious Gue'ron'sha order like yours allow for a peaceful meeting. Most encounters are...
Deathwatch: We know how these encounters end, xeno. So, what about the agreement we sent to your leaders?
Water Caste: Yes, the coalition command acknowledge the benefit of the worlds offered and we bring with ourselves the technological and biological samples requested as payment. Without safeguards as requested, of course.
Deathwatch: Good. The worlds are yours then. We will send the patents in the present solar day. Until next time.
Deathwatch: Yes, Watch Commander. Bait, line and sinker. Jericho Reach is dangerous as it is without some awakening tomb worlds, let the Tau handle it.
Water Caste: I must say, we are surprised that the forces of Chaos are willing at last to negotiate civilly. I must ask, however—what is that…thing beside you?
Chaos Lord: *Hands Water Caste a piece of paper*
Water Caste: *Reading* Hm…a Chaos Spaw—AAAAAACHSPLSHBRBLBRBLBRBLBRBL!!!!!
Dark Apostle: O, mighty lords of the Warp and true and only gods. <tap> May in these times of need thou find appropriate to <tap>SEND a signal for your faithful servants in need and grant <tap>US the means to wage war to the blind followers <TAP> of... the... <TAP> WHO IN THE WARP IS DOING THAT GODSDAMN NOISE?!
Water Caste: My pardon, Aun'ron'sha. I'm an emissary of the Tau Empire with a request of a temporary alliance against the Gue'la of the Imperium who threaten our expansion. I didn't intend to disrupt your rituals or your belief. I find those interesting, in fact.
Dark Apostle: (Looks at the xeno, then the Chaos shrine. Then the xeno again and lastly to the shrine.) ...Thanks.
Water Caste: Most honored visitor from beyond our galaxy, will you accept—
Q'Orl: *Cricket Chirps*
Water Caste: Uhhh...why are your proboscises on my pants and—OH SWEET MERCIFUL GO-ARGHGARARARALROLRAL!
Q'Orl: *Cricket Chirps*
Water Caste: Ancient race of the cosmos beyond, we, the Tau, would be honored if you—YYYYEEEEEEEEEEAAAAAAAAAAAAAAGGGGHHHHHH!
Dalek: EXTERMINATE! EXTERMINATE! EXTERMINATE!
Water Caste: The Tau would gladly welcome an empire of Gue'vesa who are open to peaceful talks rather than mindless violence.
Galactic Empire: You have precisely three seconds to identify before our ships will open fire!
Water Caste: Oh! W-we are an Empire of peac-*BLAM*
Galactic Empire: Whoops, looks like my watch is fast.
Emperor Palpatine: Yes....the force is weak in this one...
Water Caste: It would be greatly appreciated if you could inform us what the "Force" is, most honored highne-*Force lightning*-DAAAAAAAAHHHH!!!!
Emperor Palpatine: You have paid the price for your lack of vision!
Water Caste: Hello, oh great cyborgs from outside our galaxy. Feel free to enter our space, and prepare to be boarded so that we may negotiate. Borg: We Are The Borg! Lower your Shields and surrender your ships. We will add your technological and biological distinctiveness to our own. Resistance, is futile.
Water Caste We mean only peace, not harm, new fr...AAAAGH THEY ARE ON OUR SHIP!*Assimiltaion intensifies* ITS ASSIMILATING ME! HE... I am Borg. Resistance is futile.
Water Caste: Oh, the great noble Covenant, would you like to merge our coalition of Aliens together, in order to make a even stronger emp-
Covenant: Wort, Wort, Wort!
Water Caste: Well we will really appreciate if you can speak norm-ARLGRARARAL!
Covenant: Glarble, Glarble, Glarble!
Prophet of Truth: Nay! We are the true Alien hegemony! Besides your leaders seem to have stolen our designs of the Grav-Throne and Prophet's Robes. We demand payment for copyright infringement.
Water Caste: But most noble prophet, I am afraid to say that we were the ones who came up with the idea of the space pope.
Prophet of Truth: Nay that is HERESY! What you say is HERESY! Men! Prepare the Energy Projectors!
Water Caste: Okay okay okay! You guys are the true alien hegemony and you invented the concept of the flying space pope. Can we please have a trade deal? Pretty please?
Prophet of Truth: It seems like your quick thinking has saved your people from your own insolence. I will check and see about this...trade agreement with my fellow high prophets....
Water Caste: We are most honored to welcome the Slaugth to these peaceful negotiations, regardless of how outwardly repugnant you are.
Slaugth: Yessss, we are mosssst honored to sssspeak with ssssuch warm and inviting flessssssh.
Water Caste: Pardon, but why are you referring to our flesh?
Slaugth: Yesss, we speak of your flessssh. We are hungry. Ssssso very, very hungry *Surges forward like a tide of maggots* We ssssshall feassst now.
Water Caste: Please remain on your side of the roo-OH ETHEREALS, IT'S EATING ME! AAAAAHHH!!!! AHHHH!!!
Slaugth: Ssssilence your criesssss. Your flessssh sssserves greater mindssss now.
Water Caste: We are here to inquire as to why you decided to attack a world with nothing but infantry, and then burst into interpretive dance on the battlefield before suddenly walking into our POW camps, it's a bit strange.
Thyrrus: *Flashes weird colours and gurgles.*
Water Caste: ...I don't understand.
Thyrrus: *Strikes a dramatic pose and flashes completely red*.
Water Caste: *Tilts her head in confusion*
Water Caste: Hello sir Prince, we would like to negotiate colonization settlements with your people.
Exodite Prince: Are you going to trash our lands, pollute the planet and drive us out of our homes?
Water Caste: Why would you say something like that, that's a horrible thing to do.
Exodite Prince: Because it's what the humans won't stop doing, now are you going to be like them or not, I need to tend to the shrine of Isha.
Water Caste: Our technology is very environmentally friendly and we will respect your original land boundaries. Could you tell me about "sorcery"? The Ethereals are very curious.
Exodite Prince: Pssssh, you wouldn't understand even if I gave you a century, kid.
Water Caste: Kid!? I'm twenty!
Exodite Prince: Wow, you really are just a little baby.
Water Caste: Hello there, do you have a permit for your stage performance?
Harlequin: Of course, of course *shows her*, hey you wanna hear a joke?
Water Caste: Sure, I suppose I have enough time.
Water Caste: *At the hospital a day later* Ow my sides.
Water Caste: We will be honored to join with you in destroying this Chaos threat, Gue'la.
Inquisitor: YOU KNOW NOTHING OF CHAOS! YOU ARE WORSE THAN THE CULTISTS THEMSELVES! DEATH TO THE XENOS AND THEIR HERESY!
Water Caste: Please, if you would-*BLAM*
Inquisitor: Very well, we have use for you...as fodder for the heretics as they march towards you in overwhelming force.
Water Caste: Um, I request the presence of any Fire Caste cadre within the vicinity! We have an ally that is willing to help us... *Poo lands on face* And he has pets.
Water Caste: Ah, my peculiar clothed Gue'la! We are most pleased to see our overtures to meet you in this town were well received.
Vulpes Inculta: Indeed, they were. We in the name of Great Caesar wish to offer you all the hospitality we can.
Water Caste: Ah, the Thermic Lance we talked about before! It is a splendid weapon your fellow...
Vulpes Invulta: Frumentarii, that is their rank. I assure you it's a weapon that can address your short range...deficiencies. Would you like a demonstration?
Water Caste: *hands clasp* That would be wonderful! Before we begin, I wish to ask...what are these cross shaped poles you've erected?
Vulpes Inculta: Those poles? That's part of how we show our hospitality later, my dear...
Water Caste: Ah, a sentient, advanced, artificial intelligence from ages past! Elder one, have you and yours a name by which we Tau may call you?
Reaper: WE ARE BEYOND YOUR LIMITED COMPREHENSION. WE ARE PEACE THROUGH DESTRUCTION. AND YOU ARE ASHES TO BE GATHERED AT THIS END OF A GALACTIC CYCLE. *bbzzzzzzrrrrrrrrrrnnnnnnnnn*
Water Caste: We must unite or be destroyed.
*Piiiiiiiii...* *A bombardment rains from orbit smashing the Reaper.*
El'Myamoto: Phew, that was a close one. Now I must go, Water Caste.'
Or, most likely
Thousands of Reapers lay shattered in orbit, and on the planet's surface, flames of an Exterminatus dying down.
Water Caste: Huh... they were not as threatening as first expected. Regardless, thank you for your help, gue'la. However, you did not need to purge the planet, I assure you we had it under control.
Inquisitor: You know what they say! 'Suffer not the alien to live', haha... Besides, I've been waiting to use that Cyclonic Torpedo for months now! The stockpile we have was really causing my Exterminatus hand to act up... actually, now that I mention it...
Water Caste: ...huh? WAIT OH NO-AAUUUUGGH