I've got all these french language ipad apps, but I'm not as fluent in french as I am in english, so I'd like to be able to hold a word and have it defined for me. Can I do this?
I don't believe so, that dictionary is tied to the language you set for the entire phone.
When you type, you can change the auto-correct library to French when you change the keyboard, but that won't change the dictionary.
EDIT: For that matter, I don't think French is even available yet. I believe the dictionary is a subset of iBooks and only English and Japanese are available.
Apple said:
Does iBooks include a dictionary?
Yes, iBooks allows you to look up the definition of words using a built-in English or Japanese language dictionary.