It was subtle at first. Nobody noticed it. Especially not Arthur Pendragon.
He may be a great king, but he was an oblivious one. So when he let Merlin reread his speeches or write some for him – he never considered that Merlin was a farmer.
He had never learned to read nor to write. He just assumed that Hunith might have taught him some letters and that was it.

But even Arthur noticed that something about that assumption didn't add up, when he met Merlin's mother during the battle of Ealdor.
Don't get him wrong. She could write. Pretty beautifully too. She was one of the rare few who could. But there was a difference between the small mails she wrote to Gaius and the formal ones Arthur wrote for court.

The difference was the language. If that doesn't make sense to you. I will explain.
Almost every royal household spoke a foreign language.
They were very important tools when Arthur had to converse with enemies or allies. Each kingdom had it's own language for important businesses.
Camelot was a kingdom that prided itself with it's education in that matter. As Camelot was centered in the middle of Albion, it spoke each of the languages that were used by the kingdoms surrounding it.

Which was why Arthur's speeches and laws were written in multiple languages. In multiple versions or just in the one they currently needed. For example, he wouldn't write a truce treaty for Bayard in the same language he would write it for Queen Annis.
Hunith spoke and wrote in the tongue of king Cendred. There was no way she had learned any of the others.
As far as Arthur had learned, she only spoke this one and maybe a little Greek.

But whenever Arthur asked Merlin to reread and correct one of his speeches – Merlin never complained about it. Or pointed out words he didn't understand.
In the beginning, when Arthur had just wanted to made fun of Merlin for being uneducated, this behavior, Merlin's blunt corrections of his misspellings had impressed him like nothing else ever had.
Merlin was always right, when he scrunched his face like that and rewrote whatever it was that Arthur had written.

At some point, Arthur had even asked Merlin to write speeches for him. Merlin always did an amazing job at those. However, Arthur noticed that the language of said speech changed randomly. Like Merlin wasn't taking into consideration which kingdom the letter was for. Maybe he didn't know which kingdom spoke which tongue. Sometimes the speeches just changed tongues multiple times in between. Arthur used to think these were intellectual pranks played on him by his servant.

Until Arthur began talking to Merlin in different languages. Merlin switched between them like others skipped a book page. Whenever Arthur did this – suddenly the speeches were written in that same language he had chosen to talk in. As long as Arthur would supervise him, Merlin never changed tongue unless Arthur did.

It was fascinating to see how Merlin never seemed to notice language changes. He never noticed when someone suddenly switched or couldn't find the right word and just used another one of another tongue.
Merlin talked to a person the way they talked to him.

And so far, Arthur had yet to find a language that Merlin wasn't fluent in.
He had tried everything. He even learned a bit of a new language that hasn't been used in Camelot since the Fisher king disappeared.
When Arthur had just said hello in that language, Merlin began talking to him like he was raised in it. So fast, Arthur failed to keep up past the "Gwaine stole food from the kitchen again.".

At this point, Arthur was quite certain Merlin was just messing with him. Nobody spoke that many languages. At least not fluently. How he did it however, Arthur could only guess.
The funniest part was, when Arthur asked him to repeat himself. And this time used a language he was more comfortable with, Merlin just switched back like Arthur had pressed some kind of button or something.
Belittling Arthur for "not listening" or being an "unreasonable prat."

When Arthur asked Gaius about it, the physician refused to believe him. Until he began to test Arthur's theory and gave Merlin a book that was written completely in Symbols.
When Arthur asked what language that was, Gaius had told him it was Egyptian. Whatever that meant.
Merlin had skipped the pages for one minute with his eyes, before he suddenly began babbling nonsense to Gaius. Pointing at certain Symbols and asking questions that made absolutely no sense to either of them.

Gaius had just gave his signature eyebrow raise and asked Merlin what he had just said. And once more, Merlin looked displeased.
"Seriously, Gaius. When you ask me to read something out to you, you could at least listen." Then he had the audacity to look annoyed, before he finally repeated himself. And said something about a form of conversing one's body that was described on this page.
In utmost detail.
Creepy.

Even Gaius had no explanation for this.
It was as if languages didn't exist for Merlin. Like all he wanted was to talk to people and so he could. The smallest change of conversation just flipped a switch in Arthur's manservant and bam – Greek. Bam, welsh. Bam. Russian.

It wasn't until after Camlann, when he had made up with Morgana and Mordred that he could kind of grasp the idea of the answer to that question.

They were on a patrol. Not that these were necessary anymore, with Albion at peace and all that. But bandits still existed and the knights needed the exercise and experience. Just in case the peace wouldn't hold.

There was a fire in front of them and Merlin and the knights just sat around, chattering mindlessly.
"Is it because of your magic?", Arthur finally asked, head deep in thought. He had already forgotten that he had yet to explain what he was talking about.
Merlin straightened immediately. The mention of him having magic still had him acting nervous. That was simply because Arthur was still working out the details about the repealed magic ban and the fact that most people just avoided the topic.

"What is because of my magic?", Merlin turned his head to Arthur, eyes wide with anticipation. The hint of regret and fear still shading his features, like the flames did his cheekbones.

"Languages.", Arthur said, nodding fiercely, as though that explained everything. The knights bumped each other to get each others attention. This was bound to be an interesting discussion. Many of the knights had had the same questions that Arthur had. They had spend long nights discussing it, betting if and when Arthur would notice the unusual education his manservant apparently had. Not knowing that Arthur had been investigating the whole topic long before they had.

"Languages, Sire?", Merlin asked confused.
Arthur nodded. "Yes, Merlin. Sprachen(German). Linguae(Latin). Языки. (Russian). Լեզուներ(Armenish). Langues (frensh). Should I go on?"
Merlin gave him an unimpressed look. „No. I don't really need you to repeat the same word ten times over. I know what the word language means. I'm not stupid."

"Could have fooled me. So. Is that the reason?"
"Reason for what?", Merlin looked genuinely confused now.
"That you speak all of them?"

Merlin halted, thinking rapidly. "I'm pretty sure I don't speak all languages. I'm also pretty sure different language would also mean that the word language itself sounds – well – different in each one.", he said. Then he put his finger to his lips in thought. "Then again, I've never talked to anyone who speaks a different language before. I wonder what they sound like."

Arthur focused his full attention on not face palming right now. And Arthur was pretty sure Language and Sprache sounded very different. "Merlin, you keep switching between languages all the time. You must have noticed that."

Merlin frowned. "Don't mess with me, Sire. I would know if I suddenly began speaking another language."

"Really!" Arthur deadpanned. Holy crap, Merlin really hadn't noticed?
"Because I've been trying to figure out how you do it for YEARS. I say one word in french and suddenly you start talking back with full blown french. Grammar, accent and all. Gaius finds an old book in a language that hasn't been taught in Centuries? Who speaks it? My manservant who never had a tutor.
I need a formal letter for Bayard in Greek? Sure, let Merlin write it. Just make sure you order him around in Greek and don't expose him to any of the maids or else the letter changes it's language half way through. I need a translator for the bandit we've captured that nobody knows which language he speaks? No problem, whatever it is, Merlin speaks it.
So, please repeat yourself, Merlin. Tell me how you never met anyone who speaks another language. When I have purposely put you in the same room with one. Multiple times."

Merlin blinked a few times, mouth dropping open. Before he frowned again, trying to remember any of that.
"Wait – THAT's the reason you always ask me to write your speeches in your chambers?
That's why I have to repeat all the stuff the bandits say to you? I thought you were just trying to annoy me.
That's why Gaius makes me read books out loud? And I thought he was just getting blind."
Merlin rubbed his temples.

"How do I speak languages without knowing?"

"Beats me. I'll say it again. Maybe it's your magic?"

Merlin grimaced and stared at his hands. "Who would have guessed.", he mumbled and closed his fingers down to fists.

"So, you think so too? Good. For one second I thought you were actually useful.", Merlin chuckled at the jab.

"So, how many languages do I speak then?", Merlin asked, eyes beaming with curiosity.
Arthur shrugged. "I'd take a wild guess and say all of them. You'd make a good ambassador.", Arthur smiled, and began playing with his sword in the ashes of the fire.
"Really?", Merlin's grin spread even wider.

Arthur rolled his eyes.

Now Gwaine interrupted them, grinning just as happily as Merlin. Arthur wasn't surprised. Gwaine was as fond of Merlin as Arthur was himself. And he always talked Arthur's ears of to give him some recognition. Hmm. Maybe he should make Merlin his adviser or something.
It was about time.

"So, Merlin. Does that mean you could actually understand that rabbit you were talking to last week?"

Arthur blinked, almost letting his sword fall into the camp fire.

Merlin blinked. "Don't be ridiculous. Rabbits don't talk.", Merlin rolled his eyes.

Arthur breathed in relief.
"They gesture with their heads and noses. Now dogs – those like to chat."

The silence that followed after that was deafening.