Extract from the how do I FAQ
Visual Novel/Dialogues example - LINK
It's not exactly a dialog system as you intend it, since it just stores and display some text lines, not really allowing for the user's input.
Yann made an example opening a txt file and displaying lines out of it.
I'm sure he'll be glad to post it around.
For the loading time, I'm not sure hundreds of text lines will be an issue, not even sure a thousand would be.
In Space Jinx I implemented a hundred lines in actions and nothing is noticeable on the startup.
The longest thing to load/download at first is still the art of the game, not it's "binary" datas.
The dialog system you want to put in place can be tricky though. I guess for each sentence, you would have the string itself, X possible answers, a reference to the sentence the current string is an answer/following for.
I'd still stick for the array (even possibly 3D).
Or, once Yann has posted his file, the filling of an array from an external txt files.
You might also want to check the csv plugin, that might be of some help as far as serializing goes.
You might also consider the hash table plugin that provides ways to import/export keys and values if I'm correct.