I believe that Oikawa's mother is Argentinian (which is how he is able to play for Argentina) and that he can speak fluent Spanish. My mum actually came up with a really interesting headcanon for this which I now live by, so here you go!

Special thanks to Google Translate qwq

"Hey, Iwa-chan."

Iwaizumi felt a hand grab his arm as he was about to leave the gym. He turned around to see Oikawa standing behind him. The setter had seemed uncertain all day and now, it seemed, the feelings were ready to come out.

"I was, um, just wondering," Oikawa said, flushing slightly when everyone turned to watch the scene. He looked anxious and unsure, not willing to meet Iwaizumi's eyes as he blurted out, "¿Te gustaría ir a una cita conmigo?"

There was completer silence in the gym and everyone stared. Oikawa went redder. 

"Lo siento por ser raro," He rambled, letting go off Iwaizumi's arm. "Te quiero mucho y quiero llevarte a una cita. ¡Una sola fecha estará bien! ¿Qué te parece?"

Iwaizumi looked at the nervous Oikawa, and slowly gave him a pitying smile. "Oikawa," He said softly.

"¡¿Sí?!" Oikawa said, starting slightly.

"You started speaking Spanish."

The setter stared, sighed and then forced a bright smile. "Don't worry about it, then, Iwa-chan!" He said brightly, in Japanese, making the ace frown. "It wasn't important anyway!" Breezily, he strolled past Iwaizumi, humming to himself.

Iwaizumi watched him go worriedly, forehead creased in concern. The team was dead silent until Makki finally broke silence. "Since when the fuck did he speaking fucking Spanish and when was he so fucking good at it?!"

"He's always been able to speak Spanish and he's just as good in it as he is with Japanese," Iwaizumi explained, still watching Oikawa as the setter slipped into the locker room, humming. "His mum's from Argentina, so... He can speak Spanish."

"What happened just now then?" Grunted Kyoutani, with a very bad looked of feigned disinterest. "He's never broken like this before, or whatever."

Iwaizumi narrowed his eyes. "That's what I'm worried about," He answered and began to pursue Oikawa.

He opened the locker room doors, narrowing his eyes at the setter. Oikawa was, surprisingly, mostly dressed. He was finishing doing the buttons on his blue undershirt, the tie swung lazily over his shoulders. He looked up when Iwaizumi walked in and smiled.

"Hey, Iwa-chan," He began. "You look constipated agai- o-ow, hey! Iwa~!"

Iwaizumi grabbed him by the ear and shook him slightly, not too hard. When he was finished, Oikawa was pouting up at him and the team had carefully walked in.

"Iwa-chan, you brute! That hurt!"

The ace crossed his arms and glared. "Since when did you accidentally swap languages in the middle of your sentences?"

Oikawa stiffened slightly.

"I know you, 'Kawa. The most you usually say in Spanish while not speaking Spanish in the first place is swear words. What the hell was that?"

Oikawa's eyes narrowed and he straightened up, looking down at Iwaizumi. "Just because it hasn't happened before, doesn't mean it'll never happen. I guess there's a first time for everything, Iwa-chan."

Iwaizumi glared accusingly. "You never make mistakes like that, though. Not eve when you were little!"

The setter turned his back, beginning to sort out his tie. "So?" He snapped. "It was dumb anyway, so it doesn't matter. Just drop it."

"No, you're acting odd. What's the problem?"

Oikawa smoothed his tie out and began to pull the sweater-vest on next, carefully fluffing out his hair afterwards. Iwaizumi knew he was ignoring him when Oikawa started to pull on his blazer.

"Oikawa," He said sternly.

The setter looked at him, then picked up his bag, swinging it over his shoulder and beginning to walk away.

"Oikawa," Iwaizumi growled, beginning to walk after him.

"Bye, Iwa-chan!" Oikawa called, a bite to his voice, as he breezed through the gathered team, shouldering them gently out of the way to the door.

"Dammit, Tooru!" Iwaizumi darted forwards, before Oikawa could even lay a finger on the handle, grabbing his shoulder and pushing him against the nearest wall. "What the hell is up with you, right now? I know something's wrong!"

Oikawa stared at him, eyes wide and lips parted in shock. He fumbled dumbly for words, not making a single sound. His face grew red and Iwaizumi ignored a small gasp from Yahaba begind him.

"Tell me what's wrong, Tooru. Right now."

The setter stared at him, face warm, and his eyes flickered briefly over to the team. "I-Iwa-chan, can we deal with this later?"

Iwaizumi glared, not letting him move. "How 'later'?"

Oikawa flushed. "To-tomorrow?"

"Fine." Iwaizumi stepped back, leaving Oikawa hanging by the wall for a little while. Then his friend slowly peeled away, keeping his beautiful eyes on Iwaizumi until he had to leave. The door had barely closed behind him when Yahaba ran out after him, yelling for him to stop for a second.

Iwaizumi stood, keeping his eyes trianed on the wall for a little longer.

"Fuck," He eventually grumbled, turning around and walking back. The team stared after him, bewildered.

Then Watari sighed loudly. "Honestly, Iwaizumi-san, at least take him out for dinner before pulling such a bold move."

"I will throw a shoe at your head, if you don't shut up," Iwaizumi answered.

.:*:.

The next day, it happened again. It had honestly slipped Iwaizumi's mind, since he had gone with a group of his friends to watch a late-night basketball game and had come home early in the morning. So, the last thing he was thinking of was Oikawa's first lingual slip-up. That is until it seemed to happened again...

"Iwa-chan!" Oikawa called as he walked into the gym. "Can you start leading stretches for me?"

"Aren't you suppossed to be captain?" Iwaizumi answered, rolling his eyes. "Stop being so lazy, you loser."

Oikawa pouted. "Aren't you suppossed to be my vice-captain? Stop being so lazy, mi amor ~!"

Makki spluttered, choking on his own spit, most likely, and Iwaizumi gave him a brief, curious look, before turning back to Oikawa. "Okay, are you okay for real?"

"Hmm?"

"You just mixed up your Spanish and Japanese again, Shittykawa. Is your brain okay?"

Oikawa looked at him, with an expression that could've been disbelief. Then he beamed. "I'm sorry, Iwa-chan. I meant to say, 'Stop being so lazy, you ugly porcupine-"

Iwaizumi hurled the volleyball at him without a second thought and laughed at Oikawa's offended yelp. "Iwa~! That hurt!"

"Good! I'll lead the stretches, Crappykawa, just do whatever you need to do!"

Oikawa beamed at him. "Okay, then, mi amor, I- ow, don't throw another one! I'm going, I'm going!"

Iwaizumi rolled his eyes, approaching the team. "Guys, you heard him. Get over here, we're starting now. Partner up."

As the team listened to his instructions, he was incredibly confused as to why Hanamakki completely ditched Matsukawa, leaving a very offended boyfriend in his wake, and approached Iwaizumi. "Wanna partner up?" He asked promptly.

The ace looked him up and down, concerened. "What the hell have you done?"

"Nothing, I swear," Makki promised. "I just... Need to ask you something."

Iwaizumi sighed and followed Makki's gesture to sit on the floor, letting the other spiker push down on his back to help him stretch.

"Can you speak any Spanish?" Was Makki's first question.

"Barely," Iwaizumi answered. "I've picked up that 'sí' means 'yes' but that's about it. Oikawa swore to himself that he'd never teach me."

Hanamakki pushed down a little further. "So you don't know what 'mi amor' means, then?"

"Well, Oikawa said it was an insult, so," Iwaizumi shrugged. "I'd say I've got a vague idea." He heard Makki sigh and shot him a look. "Why are you being so weird about this?"

The other spiker returned the look, but it was carefully neutral. "I just wondered if you'd teach me Spanish," He said evenly. "I'll have to ask Oikawa instead."

Iwaizumi didn't bother pointing out the obvious lie. Instead, he turned properly again and continued stretching. "Get ready for a, 'no', then."

.:*:.

"Tus ojos son tan hermosos," Oikawa said to Iwaizumi, leaning on his shoulder one day, looking intently into his face. "Creo que estoy perdido en ellos..."

Iwaizumi grunted. "No one understands you, Oikawa," He said, brushing him off.

Oikawa smiled. "And that's the fun part!"

"Well, it's not fun for me," The ace growled, closing his eyes so he could block out his pretty friend. No one had the rights to be that pretty... "Can't you just speak Japanese? I don't understand Spanish and you know it."

His friend shrugged, turning to open his locker indifferently. "I can say whatever I want, Iwa-chan! Besides, it's nothing that important."

"Yeah, but it's still annoying when your usual dribble starts sounding like you're just spouting random noises," Iwaizumi grunted. He wasn't upset Oikawa was speaking Spanish. He just like knowing what his friend said. He liked being able to hear his voice. He liked being able to listen to him. He liked him.

See? He could admit it. Maybe never to his face but he had long since come to terms over his giant crush on his beautiful friend - it had been an interesting way to start junior high school when he found out, anyway... Iwaizumi was completely done with the whole 'I'm totally straight and this is just hormones' act he had put himself through for two and a half years. He was far happier with 'I'm Oikawa Tooru's best friend and I secretly love him', the mindset he had settled into just as middle school drew to a close. Granted, it was annoying how Oikawa always seemed to get a natural glow-up every time Iwaizumi blinked, but at least he was still there. 

Any lighting made his eyes sparkle. Even in darkness, there was always a twinkle of starry light in them. His hair tickled Iwaizumi if it ever brushed his skin, constantly looking so soft and silky, and seemed made to run fingers through. Not to mention, he had never managed to get any acne, leaving his skin perfectly smooth. Kissable lips, kissable neck, kissable hands, kissable face... Every part of him was beauitful to the very infatuated ace, who was definitely a simp.

Gods, Oikawa was pretty... but sometimes Iwaizumi really wanted to shut his pretty mouth up.

Like now.

"Spanish is not random noises!" Oikawa spluttered. He had been trying to defend his second language furiously as they walked to their physics class, violently gesturing with his hands when he tried to speak. "It's a beautiful language, how dare you disrespect it!"

"All languages are random noises," Iwaizumi countered, with a triumphant smirk. "How about that?"

Oikawa scowled, pouting. "No! Languages are a really complex, systematic-"

"Jumble of sounds," Iwaizumi interupted.

"No! No, no, you're so wrong, here!" Oikawa insisted. He sighed, sagging. "Eres tan irritante, mi hermosa amigo..."

Iwaizumi flicked his forehead lightly. "Stop rambling, Shittykawa," He joked. "You sound like an ugly, old madman."

"Oh, you did not just say that! Iwa-! Iwa-chan! Get back here, you idiot!"

.:*:.

Iwaizumi was very much done with his teams' bullshit and he was also very much done with Oikawa.

It was Tuesday, morning practise, and Oikawa hadn't spoken a word of Japanese to him. It was all just Spanish and the team was finidng it hilarious. Iwaizumi was not finding it as funny.

"Oi, Crappykawa," He barked, storming over to his friend.

Oikawa smirked. "¿Sí?"

"Can you stop that?"

"¿Para le qué?"

Iwaizumi scowled and Oikawa grinned. "Oikawa," He said warningly.

"Mi amante más preciado," Oikawa answered quickly. 

"Are you insulting me in Spanish again?" Iwaizumi frowned, confused. Yahaba and Makki started laughing harder, Yahaba actively having to lean on Kyoutani to remain upright while Makki let himself fall ungracefully onto the floor. Iwaizumi flushed angrily. "Just look in the mirror if you're running out of creative insults, and you'll see the world's vainest shitbag."

Annoyingly, his own insult didn't have the intended effect.

"¿Por qué insultaría a alguien tan atractivo?" Oikawa said, looking Iwaizumi up and down with an indescriable gleam in his eyes that made Iwaizumi want to shiver.

He didn't shiver, though. He just crossed his arms, struggling to have a comeback to an insult he didn't even know. "You're the alguien tan atractivo," He answered back eventually, knowing he sounded stupid.

Hanamakki wheezed and Yahaba made a chocking noise. Oikawa dropped the volleyball in his hands, staring at Iwaizumi wide-eyed and red-cheeked.

"Uhm..." Oikawa fumbled, quickly ducking his head as he made a fuss about scooping up the volleyball. "No sabes de lo que estás hablando," Oikawa muttered, not looking up.

"Say it again!" Makki insisted, sounding like he was close to death with laughter. "Say it again!"

Iwaizumi hesitated but then Yahaba started yelling, "Call him 'bonito'! Call him 'bonito'!"

"Yes, yes!" Hanamakki urged. "Say it, please!"

With two teammates excessively hyping him and a childhood friend turning redder by the second, Iwaizumi put his hand on Oikawa's shoulder and smiled at him. "Bonito," He said, maintaining eye contact.

Oikawa pushed his hand off quickly, turning away and hiding his face. "Unfair!" He hissed, finally speaking Japanese again. "That- that's against the rules! You can't- you can't say that!"

"Why not, bonito?" Iwaizumi said, beginning to enjoy the nice change in power. "Do you have a problem with it?"

"Kinda!" Spluttered Oikawa, beginnng to lose his composure. "You-! You can't just-! That's not-!" He turned on his heel. "Coach!" He called loudly. "I'm going early!"

The coach, who had been watching the entire event, just waved his hand vaguely. "Off you go, then. Iwaizumi, try not to crash your captian during practise, he's useful sometimes."

"He started it," Frowned Iwaizumi, as Oikawa began to quickly make his way off.

"And I'm finishing it. Goodbye, Oikawa!"

"Bye," was the quick response.

"Bye, bonito," Iwaizumi called after him, just before the door slid closed. Everyone could hear the strangled-sounding screech that emitted from outside the gym with ease.

.:*:.

"Iwaizumi-san," Kindaichi asked, nudging the ace carefully. "I've got a question..."

"What's up?" Iwaizumi asked, tossing volleyballs into the trolley as the Seijoh team cleared up the gym at the end of Friday after-school practise.

Kindaichi swallowed, shy. "I was wondering how to, um, ask a boy out..."

Iwaizumi stopped, and looked at him. "Is this about Kunimi?"

Kindaichi blushed a little. "Maybe."

The spiker thought over the question. "I think... being honest to him and yourself is the best way to do it. If you're gonna ask, there's no use testing the waters when the answer will always be the same."

"Well, what do you do if they say 'no'?"

"Give them a bit of space," Iwaizumi replied. "They're probably gonna need a while to accept it and so will you. When that's over, you'll still be able to be good friends afterwards and, if that doesn't happen, you'll find better friends."

Kindaichi hesitated. "And... if he says 'yes'?"

Iwaizumi shrugged. "Take him out on a date. Nice and simple."

"Is that what you did with Oikawa-san, then?"

Iwaizumi nearly froze. "Huh?"

"You and Oikawa-san. You're both dating, aren't you?"

"Kindaichi," Iwaizumi said slowly. "Oikawa and I are just friends."

Kindaichi frowned. "But... he's been flirting with you for a whole month?"

"How?"

"In Spanish. He's been flirting with you."

Iwaizumi stared. "For a whole month?!"

Kindaichi paled. "Oh, gods, you had no clue..."

"Tooru!" Iwaizumi shouted, ditching the volleyballs and his underclassman as he sprinted towards the setter.

"Huh?" Oikawa spun round, concerened. "Everything okay? What's the matter-?"

"Have you been flirting with me for a whole month?!"

Oikawa froze and the colour drained from his face. "No," He lied, weakly. The entire gym had gone quiet.

"But you have, haven't you? I-In Spanish, you were flirting with me!"

The setter stared at him, then slowly bowed his head. "I'm sorry," He muttered. If a single person had moved, no one would've heard him. But it was silent enough to hear a pin drop and everyone heard the soft apology. "It was selfish."

"Why would you do that?" Iwaizumi asked.

Oikawa swallowed, tearing up slightly. His lips moved but not a sound slipped out.

"Porque me gustas..." He finally whispered.

"...What? I don't understa-"

"Because I like you! Porque me gustas! Me gustas, me gustas, me gustas!" Oikawa shouted. He really did look on the verge of tears now. "And you can reject me! Humiliate me! I-I don't care! It never mattered anyway!"

Iwaizumi tilted his head. "Why would I reject you?"

"I'm annoying! And a bother, I'm selfish and petty and-"

"Your pretty mouth needs to shut up," Iwaizumi growled over him.

Oikawa blinked, confused. "My wha-"

Iwaizumi kissed him, hard, on the mouth, with five and a half years of waiting finally coming to a very satisfying conclusion. Oikawa's fingers on his back, chest to chest,with Iwaizumi's hands on his face. Iwaizumi pulled back.

"There. You've shut up and it's only taken eighteen years."

Oikawa blinked, still mildly resembling a confused pet. "I don't... understand-"

"Next time, tell your very in-love best friend that you like them and don't worry about getting rejected when you're absolutely flawless, okay?"

"Hajime..."

"What?"

Oikawa stared, then smirked slightly. "Necesitas mejorar en besar a la gente."

"Shut up with your bloody Spanish."