I usually think in English but, depending on what it is that I am thinking about, I would, sometimes, think in another language. For example, if I am thinking something about, say, animals, I think in English because English is the language in which I acquired most of my knowledge on them but, if I am thinking something about my family, I do it in Spanish while French and Italian usually pop up just as a phrase here and there -kind of like an interjection, a commonly used phrase in that language or something that has a special kind of meaning that cannot be translated properly.
As to parrots, I think they learn not so much words but the whole sound as an unit... sometimes they do it in context, sometimes because they simply like the sound of it and, sometimes, they associate it with something else. For example, Naida BFA only says one thing and one thing alone: "Hi, Nate!" (she was named Nathan in her previous life). It seems to me that she thinks the whole thing is her name and always uses it when she wants to call attention to herself - like at dinner time, she wants her seeds/nuts in a hurry so she always says it in the evening several times in a row. Because this is the only thing she says, I always repeat it back to her after she says it but this created a bit of confusion for Sweetpea because he now appears to think that Hi, Nate also means either Nite Nite or dinner because as soon as I come downstairs to feed them dinner, he starts saying: "Nite nite - Peanuts - Hi, Nate"

I guess he wants to make sure that I understand what it is he wants so he uses all the sounds he knows that would apply to the situation. Which brings me to the motivation behind their learning to talk human which, in my personal opinion, is their desire (need?) to communicate.