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The unseen servant is mindless, so it doesn't understand language as such, only the caster's ability to give directions that is part of the spell effect. Directing or redirecting an active spell is a move action that doesn't provoke AoO. So much for the RAW, as far as I can see.
I think I'd say it is not a purely mental action, because it's defined as a move, but the caster can direct the spell using either gestures or speech (in any language that he knows - his intent is most important).
Why couldn't it be directed by simple mental instructions?
The description of Unseen Servant says "An unseen servant is an invisible, mindless, shapeless force that performs simple tasks at your command."
The description of Animate Dead says "This spell turns corpses into undead skeletons or zombies that obey your spoken commands."
It seems these are 2 different kinds of control: the first sounds generic (it's similar to the mental control of a spiritual weapon), the second is explicitly verbal.