they both speak a very similar (but slightly different) Japanese dialect...
I really wish i had them so could find out more :S
I have them both, already did their fate ep, but didn't paid much attention to it, I think that I can see it again from the character page, if I can, I'll record the ep and post it here, from what I remember Socie is a really timid person, while Yuel is the opposite.
Ok, the story is a bit convoluted but here is a rough run down.
Firstly, they are childhood friends who were reunited by chance on the Grancypher. They are also both descendants of a thousand year old "Fox Kingdom" - a particular ethnicity of Erun with tails and an affinity for fire magic.
Yuel is on a quest to find other descendants who can use fire magic and wants to restore the kingdom. Socie knew about this, but kept the fact she could use magic hidden to protect Yuel.
It turns out some mystery faction is hunting people from the old kingdom. Socies' parents knew they were being hunted so sent her to live with Yuel in safety. They were killed by the faction shortly after. Socie didn't want anything to happen to Yuel so didn't reveal she could use magic till now.
In the end they decide that no matter who comes after them, they will be safe together (esp with the rest of the ship) and the Socie teaches Yuel her fire dance.