It started innocently enough. Truly, it did - they were being prepped for a mission in Russia together. Clint was assigned to play the "bodyguard" role for Natasha (like the woman needed a goddamned bodyguard, Clint thought derisively), and she would be playing an aristocratic woman with a taste for illicit trade. The mission was simple - Nat would pose and smile and flirt her way to the arms supplier, and then she'd take him out, only after finding his international contacts, which would be quickly eliminated as well.
The mission went well enough. Natasha's native tongue sounded good on her, as Clint soon realized. And as she soon came to notice, Clint looked, well, damn good in a suit. But Clint not really having a full grasp on Russian didn't deter the mission in the slightest. He wasn't expected to speak by the high members of society when viewed as a service that Natasha had hired. And that was fine with both of them. It meant that when they spoke, they spoke in English, and the conscious language change made them more aware of what they were saying.
But when the mission was done, Clint looked at Natasha one day and said, "I want to learn Russian."
She had no idea where this came from, but when asked, he said he wanted to know his partner better, and learning her language from her felt like a productive way of doing it. She agreed, and so on the nights they didn't share workouts or missions, Natasha was teaching Clint her mother tongue.
One of the first phrases he spoke to her was "I can't live without you."
From then on, he took to calling her by nicknames in her language. Both of them preferred it this way - they didn't speak this way in front of S.H.E.I.L.D., but when they were alone, it suited them fine.
Then New York happened, and the Avengers Initiative became a reality.
At Tony Stark's initial proposal of the Avengers all living in his newly rebuilt tower, Natasha and Clint looked at each other and gave a quick "No way in hell." in a language they knew Tony Stark had no knowledge of. (Clint was later swayed with promises of a shooting range set with arrows. Natasha was taken in when Tony promised to invent a magnificent weapon for her to play with. Fury himself was a bit frightened when she said she wanted input on the design.)
Living with the Avengers has its own set of perks, really. None of the others spoke Russian, so the two agents were free to communicate with each other as they pleased. (Tony, when he first figured out what they were doing, asked JARVIS to translate. When Natasha heard this, she pulled Tony aside for a quick conversation. When they returned, Natasha was smirking and Tony's face was gaunt. She turned to Clint and told him, in their way, that he wouldn't dare think of having JARVIS translate their conversations again.)
Neither Natasha nor Clint knew whether Thor spoke any modern day Midgardian languages except English. They soon got their answer one morning when the three were seated in Tony's kitchen. Clint looked up from his coffee and said to Natasha "Your beauty drives me to madness." Thor asked for the pop tarts. Unsatisfied, (after all, proclamations like that may be common in Asgard) Natasha looked at Clint, and said very clearly, "Having you between my legs drives me to madness." Clint nearly spat out his drink. Thor didn't even blink.
And so began a new era in their stolen Russian world. Whenever one of the pair did something extraordinary (or extraordinarily sexy), the other would be quick to inform the other, in acute detail, how they felt about it.
This was fine for them. In fact, both rather enjoyed that they could be so candid with one another. They never suspected they'd be caught. They certainly never suspected who by.
Loki had been imprisoned in Asgard for three years. None of the Avengers, save Thor, thought this was a long enough punishment. But during the last eight months of his brother's banishment, Thor kept insisting that Loki had changed back, that he was the brother he remembered, to anyone who would listen. Thor wound them all down to the point that when he introduced the idea of Loki coming to stay for a few weeks, though initially objectionable, they all eventually agreed (some much more reluctantly than others).
As it was, Loki was a fine houseguest, if not suspicious at first. Who was to say that he didn't have any kind of plan that involved gaining their trust? But they had no way of knowing for sure, so all but Thor kept their distance.
Bruce was the first to crack. Loki spent all his time reading books he had brought over from Asgard. Call it curiosity, or even empathy for another outcast, but Bruce asked Loki what he was reading. The subdued answer of Asgardian spellbooks did little for Bruce's scientific mind, however, and from then on he, and eventually Tony, would spend time with Loki, asking about Asgardian culture (Bruce) and "That magic/science/call it what you will I want to KNOW" (Tony).
Natasha and Clint nearly forgot Loki was staying with them. Nearly. They'd never completely get rid of that itch in the center of their back that said "Careful. Danger. Watch out." However, they were able to continue their habit of Russian exclamations to each other without the slightest hint that Loki had any clue what they were saying, which suited them fine.
One day, though, the Avengers-plus-Loki were crowded together in Tony Stark's lavish home theater, for what Tony called movie night and Steve called team bonding. As movie options were sifted through, cries of dismay and cries of happiness were elicted at each title offered. The pattern continued until one title was offered that made Natasha look at Clint and tenderly say, "It's like what happened in Budapest." The rest of the team continued as they usually did, long used to such foreign expressions from the agents. But Loki looked at them strangely until Clint asked a sharp "What?" Loki just shook his head and said, "I was just wondering what happened in Budapest." That was enough for the rest of the team to warrant turning their heads and looking curiously at the threesome, but Loki continued. "I know the film, and the events don't really warrant such a fond tone." By now the rest of the team was openly staring, but Natasha and Clint felt the only other person in the room was the dark-haired Asgardian.
Seeing the looks on their faces, Loki quickly backpedalled. "I'm sorry. I see I've overstepped a line I had no intention of crossing. Please, don't feel obliged to answer." But the damage was done. Clint quickly jumped in, with "You've known what we were saying all this time?" while Natasha added her "When did you learn Russian?" Clearly uncomfortable under the scrutinizing stares of the assassins, Loki murmured that he had taught himself the languages of several realms while his brother was on hunting trips with the Warriors Three. "Have you understood us all this time?" Natasha asked. When she received a hesitant nod in answer, she gave a curt nod back and firmly fixed her gaze on Tony's face. "Have you picked a movie yet?"
The next day, Natasha visited Loki in the corner he'd effectively claimed during his time in Stark's tower. It was a comfortable, open space, surrounded by old books in languages the master spy herself didn't recognize. She sat down a few feet away and looked at him. Loki was sure her relative proximity was not out of arrogance, but certainty that they would not come to blows. He was silently relived that she wasn't looking for battle. It had never been his strong suit.
"If you've always known what Clint and I have been saying to each other, why have you never made it apparent?" Ah. Well. Of this, Loki wasn't sure himself. When he told her so, he received an arched eyebrow and a slight scowl at the corner of her mouth. He immediately tried to think of how to put how he felt the first time he heard them using Russian into words.
"He called you his sun, the first time I heard it. At first I processed it as dull romance, but when thinking on it again I realized it was spoken in a way that the others would not understand. I became curious at this, and wanted to know why you didn't want your teammates hear such frivolities. I thought them but trifles, simple musings of romantic minds. But then I recalled you telling me that love was for children, and I became immediately enamoured with your strange way of giving such assurances to one another. I was curious, and I wanted to know more, so I decided not to give any sign that I understood you. As you continued, I noticed that you never used the world love. An elegant way of cheating the system, Miss Romanov. I respect that. It didn't take me but a day to realize that the two of you were in love, and that both of you realized it, but never said it.
"Well, you did. But not with the word itself. You told each other your love with the words you chose to use. Using the words no one else would understand, it gave you a private world, a lover's world, to which you escaped every time you used them. When I realized this, I did not want to take it from you. I am sorry that I did. But I would like to say, Miss Romanov, that as the one called Liesmith, I am familiar with words and how they are so often misused. I would like to tell you, and your lover, how this time, you've used them they way they were meant to be."