"Oh, look, a lettuce leaf! And it's all fresh and green! I'm digging in!" - "Aww, thanks for preparing a new piece of that delicious apple for us! We really appreciate it!" - "You bought new sunflower seeds! I love those! Will you feed me some, please?"
These are some of the phrases I first thought about when I read the question. Any pet owner would love their pets even more if they said something nice about their care for them, and so would I. But after some more deliberation about the topic I realized that these sentences in now way represent the way our pets really behave towards us. Because I am sure that if they could talk, censorship would have to be introduced for animals as well, at least for the sake of keeping children's minds innocent...

Fischer's Lovebird (Agapornis fischeri)

We own two indescribably cute lovebirds; they are in fact so cute that sometimes when I watch them I have to concentrate to not turn into something resembling an insane clown on drugs and then die of shock or implosion from an overdose of cuteness.
But there exists a moment when all of that fluffy, bouncy cuteness suddenly vanishes and magically transforms into something taken directly from an 18+ rated movie (if birds watched movies, that is) - the moment when they open their sharp little beaks.

Nelli, the male bird of our pair, is a huge glutton. He sits there quietly and behaves well, but if there is just the slightest hint of new food being brought for them somewhere in the air, he suddenly goes nuts and rampages through the cage like it was his last meal, ever. He jumps up and down and flaps his wings frantically, turning around in circles and falling off the little branches we put in there, all while shrieking some incomprehensible bird speak in a volume surpassing that of an airplane. Now, since this question is about what he would say if he could talk, it would probably translate into something like the following:
"Oh my GOD why did that take so long?! I am f*cking STARVING in here, you imbecile! Give it to me now or I'll rip out your heart bit by bit and make you bleed to death!"

But Nelli is still nothing in comparison to Koko, the female lovebird. You know the proverb "Don't bite the hand that feeds you"? Yeah, well Koko doesn't care about that. She is a very curious and observant bird and wants to know about everything - especially what everything tastes like. That's why she would rather bite your finger until it starts bleeding and becoming numb than eat the food you want to give her, because she already know what it tastes like, contrary to your finger.
But sometimes, there is something even more weird about her behavior. Sometimes she just sits there and does nothing and then something in her brain suddenly makes that famous *click* sound and the insanity switch is activated, making her scream and shriek and complain in her infernally ear-deafening high-pitched voice as if there was no tomorrow. I have no idea why, but for some incomprehensible reason, she just does that from time to time. And it always lasts about a quarter of an hour, forcing everyone to leave the living room to avoid damage to their eardrums. It sounds like a huge fleet of Airbus A380 planes all taking off at the same time while someone is throwing frozen birds into their engines, making them explode and burn and crash violently into a hospital, watched by an entire primary school's pupils who of course start to cry because they are horrified and then their teachers shout at them to shut them up. Yes, it's that bad.
A translation of Koko's tirades would certainly look like this:
"What the f*ck is this sh*t garbage goddamn ***** and ***** **** the hell **** **** **** **** ********* ** ***!" (You are now allowed to replace those asterisks with any bad words coming to your mind. Oh yes, it is that bad.)

And that's why our pets would need to be censored if they could talk.
(By the way, no, we don't mistreat them. They are just that eccentric. They have always been like that. But they are still incredibly cute.)

I am the kind of person that always thinks through their words several times before saying anything and that meticulously ponders every possible way they could be understood to avoid saying something offensive or embarrassing.
I really try to live up to the proverb "Think before you talk" (at least it is one in Germany: "Erst denken, dann reden!") and would therefore rather not say anything at all than say something I will regret afterwards.
Because of this attitude of not letting my emotions control my mouth, real foot-in-mouth moments have only happened rarely to me - well, until some months ago that is.

We two together

In my German and English lessons there is a guy nobody seems to be able to relate with. He always appeared unapproachable to me and as if he would just want to be left alone and not talk to anyone. During the first two terms of my final two years at my school I did not really care about him as I simply did not know him. I even remember sitting in the classroom during lunch break and looking at him on the other side of the room because I desperately tried to remember his name - and usually I am good at memorizing names.
But then, during the third term, someone had suddenly rearranged all of the desks in the classroom and we had to choose new seats, which is how I ended up sitting next to him. After a few conversations with him it finally dawned on me that there was nobody who could relate with him because obviously no one had ever really tried to - sitting there next to me was one of the most ingenious persons I was ever able to get to know. He had a totally different and more interesting and unusual way of approaching things than any other person I had met prior to that, which is also probably the reason why a lot of people were put off and did not want to talk to him, but to me it was like a revolution of thoughts - he made all of them just spill out of my mind in cascades and then took a huge figurative spoon and stirred them so much that they started to form in a completely unknown pattern to me which made more sense than anything else I had ever thought about. I was fascinated. Soon, our conversations became a lot longer, until they would eventually last the whole time of the lesson we had together. We talked about everything that came to our minds, be it politics, the weather or whatever.

By and by he became a very important friend to me, but even though I now knew a lot about him and talked to him often, he still seemed kind of unapproachable and withdrawn (he still does that even today), which made me sad as I felt connected to him in a way.
Then one day after school we met coincidentally at the entrance of the school building which was a situation that had never happened before because he usually rushes home after school in a hurry for some reason. So when we saw each other, we just said "Hi" and then, without the need for any additional words, went home together as if it was something we would do every day. As if it was the most ordinary thing in the world. And maybe it was nothing unusual, but to me it was like some talent scout coming up to me and suddenly telling me I had been chosen as the main actress for an extremely anticipated movie. It was not usual at all. Not in any way.
At that time he also ironically had exactly the same way to school as I did except that his was a little longer and because of that we were walking next to each other for a really long time. I desperately tried to keep up the conversation with him and to avoid creating the feared "awkward silence" but for some reason I suddenly was so nervous that I only talked about strange things that obviously confused him. It had been such a long time since I had last walked home together with someone and especially with a boy I really liked (in fact, this was been the first time), so I did not want to put him off and became even more nervous.
When it was time to say goodbye I did not really know what to do because it was so much fun to be with him and I didn't want him to go yet, but of course I could not tell him because that would have been really awkward, so I just stood there staring at something in the distance to avoid making eye contact.
But then he did something I would never have expected and accordingly I was not prepared for it. Without any warning, and I really would have needed one, he gave me a long and close hug. I think my brain exploded the moment I realized it. I was totally puzzled and my mind just went blank for a second. What did he just do? Why did he do that? What was happening? I didn't understand anything anymore, just because of his hug. The rest of that situation passed by in a blur. We both said "See you" and I watched him walk away after crossing the street, feeling totally dazzled and confused and as if I had taken several drugs at the same time (at least that's how I imagine their effect ever since).

The next day after school, for some utterly strange reason, we met up the exact same way we had the day before and it seemed like that situation would repeat itself which nearly caused my head to explode again, but then he suddenly stopped walking and said something like "Well, I wanted to go buy something today, so...", which meant that he would have to go into the opposite direction of where he'd usually go. In that very moment, something in my brain definitely made the sound *click* and it swichted itself off to prevent me from thinking about what I was about to say. Immediately after he had finished his sentence, my mouth formed the most embarrassing syllables it ever did in its whole existence, as consulting my brain was not possible anymore.

"Can I come with you?"
I frantically threw these words at him but while speaking I already realized how embarrassing they sounded. I didn't even want to say that! It just came from somewhere in the depths of my subconsciousness and when it saw a fitting moment, it made a kamikaze jump right into my most important friend's face who was now totally confused.
My words also initiated the "awkward silence" that I hate so much for some seconds, which I then broke in order to save the situation but which in fact made me do even more embarrassing things.
"Nevermind!" I nearly screamed at him, being totally freaked out at my own behavior. "I'll go home now, bye!" Not even waiting for any response from him, I turned around and rushed off as fast as I could, leaving him there with the most puzzled face ever and probably thinking of me as a complete idiot.

I was embarrassed for the whole week because of that stupid incident and had to concentrate to keep me from hitting my head against any random wall I would find.
Fortunately, he never mentioned it again and he still talks to me like he did before. I am still impressed though because he is the only person who ever managed to make my emotions control my mouth before I could even think about it. In my opinion, that deserves a special mention. And in hindsight I am rather grateful for this as it has taught me to keep in mind how my emotions are still there, now matter how much I try to hide them behind the words I choose. Sometimes they try to force their way out...

Thank you anyway, Mr. Brain-Explosion! :)