Human Fluttershy’s favorite language is Esperanto because she believes in the dream of mankind uniting not being divided and angry by language.
Under Twilight’s rule, an attempt at a universal secondary language like Esperanto was attempted. The School of Friendship even made it a class.
It was called Amiceko.
Argyle knows traces of this language and while Sunny while not as fluent as her father uses some Amiceko phrases.