Italics means it is a translation

It was a cold afternoon, so the guys had decided to return to the common room. They found Remus was already there, with some sort of book in front of him.

"Whatcha doing?" Sirius flopped onto the couch beside him.

"Reading, obviously," Remus replied.

"Reading what, though?" Sirius started peering over his shoulder. "Is that another language learning book?"

"Possibly."

"That's what, the third one this year? Why would you wanna be stuck in here learning these stupid languages, when you could be spending your time with someone as wonderful as me?" Sirius shut Remus' book, losing his place. A spooky smirk cracked on Remus' face.

"Mutatione Linguarum," Remus said, with a flick of his wand. Peter, James, and Sirius looked at Remus with worry on their faces.

"Was hast du getan? What have you done?" Sirius looked shocked at what came out of his mouth.

"Moony, what's he saying? What'd you do?" James looked between his two friends.

"Just a little language spell. See how useless learning languages are now." Remus' smirk widened.

"Wie meinst du? What do you mean?" Sirius looked unsure like he didn't know what to do.

"You are stuck speaking a language that I've mastered, allowing me to demonstrate how useless it really is."

"Das ist nicht lutsig! This isn't funny!"

"Oh, on the contrary, I find it hilarious." Remus returned to the book he had been reading.

The next morning, Sirius turned to James, forgetting the previous night.

"Guten Morgen. Verdammt! Good morning. Damn!" The other boys just laughed.

"Having trouble, Padfoot?" Now Remus was just teasing his friend.

"Moony, mach das sofort rückgängig! Moony, fix this immediately!"

"He says I'm the most amazing person in the world." Remus just smiled as he got dressed.

"Das ist nicht das was ich gesagt habe! That's not what I said!" Sirius was attempting to make a retort.

"And he likes me better than you two," Remus continued.

"Ich hasse dich. Du bist ein Arschloch! I hate you. You're an asshole!" Sirius was red in the face, visibly flustered.

"Aw, Padds, I love you too." Remus just laughed as he left the room.

Sirius just sighed in frustration. 

All through morning classes, Sirius kept being called on, only to receive detention. After accumulating three detentions, Sirius had enough.

"Tut mir leid. Du hattest recht und Sprachen sind nicht nutzlos. Kannst du jetzt bitte den Zauber rückgängig machen? I'm sorry. You're right and languages aren't useless. Can you please now reverse the spell?"

"See, was that so hard? Mutatione Linguarum. There, back to normal."

"Nunca voy a burlarme de ti otra vez. Mierda! I'm never making fun of you again. Fuck!"