Yes, I do think he would have. He was a diplomat as well as a patriot, and he was a pretty intelligent person even at his most stubborn times. Besides, being a patriot does *not* necessarily mean refusing to speak the language of your (former) occupier, in real life at least.
And those who don't think he'd have spoken Centauri in S5 - just what language would he have spoken to the snotty Centauri minister in the Centauri court when he went there as Londo's bodyguard? English? Narn? I doubt the snotty Centauri minister would have understood either. I even doubt the snotty Centauri minister would have been fluent in Interlac.
Besides, there is a certain satisfaction for a person with a history of occupation to "outdo" the former occupants, to show them that he's better than them. Being able to speak their language while they can't speak his can be part of it, showing the occupants that they're simply not as intelligent, not even being able to learn another language.
It all depends on the tone of voice, and of course the general bearing of the person. If the person acts humbly, like a former slave, it doesn't matter what language he's using. If the person acts proudly, it yet again doesn't matter what language he's using.