Le Chiffre Indéchiffrable
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The title is the French name for the so-called Vigenere cipher, but here the plaintext and ciphertext are Unicode strings. Using Unicode code points as the numbers, see if you can use the first key to get the first plaintext.
Cracking the remaining ciphers without the key may be easier than it seems. The Wikipedia page for the Vigenere cipher describes one method to try and figure out the key length. Another trick is to try and exploit the fact that the plaintext probably has NEWLINE and SPACE as the lowest numbers.
If lyrics like "don't let me hurt the camels" seem nonsensical to you even after translating, that's to be expected. Try searching around on Google to find where the lyrics are from.
Look at the first letters of the 12 involved languages for a hint about how to get an episode number for each of the identified songs.
Each of the keys corresponds to a Unicode character.
Each key is the translated name of a Unicode character such as NULL, while each song has an episode number given by the Malinda fandom. Decrypt one last time to get the six-letter answer!