For a couple of lifetimes by this point Victor has considered various dialects of French to be his "native tongue." Few if any members of his current crew speak it, though, so when French is heard aboard the Sibylla it's mostly Victor talking to himself.

He does not speak Catalan, which in theory would help him communicate with Guillermo. However Guillermo communicates just fine without any words at all so it's mostly a moot point.

Guillermo is an educated mule, however, so it's not inconceivable that he understands Victor's sotto voce utterances. This would seem like a solid opportunity to inject a conspiracy, but I think that Victor's too savvy to either (1) assume that Guillermo's stupid because he's mute, or (2) be that lax in operational security. Assume that if he says anything at all, he does so with the awareness that keen ears might be listening.

If anything Guillermo's silence is what keeps the two men close—in a "friends and enemies" sense. Mina, too, seems smart enough to know when to keep her mouth shut, but the fact that even Victor can't always tell where she goes at night compels him to keep his eye on her when he can see her.

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