I don't think so...Maybe?
I guess it would depend on if Yeerks themselves could speak Galarad and if they could speak it through their hosts, rather than having to rely on their host's native tounge.
If Yeerks couldn't speak or understand Galarad when in control of their hosts, then with the flow of new Controllers coming in almost constantly, it would be natural for Visser Three to resort to using English when in human morph.
Also, Andalite might be a fairly complex language to speak aloud, especially when trying to use a human mouth. This speculation comes from the fact that the Animorphs originally called Aximili Ax because the name was so complicated to pronounce. Either that, or he said it really, really fast.
I'm just speculating here. Feel free to rip my logic to shreds with better, more thought out logic.