Is this a dialect?
 

Excited Japanese Girl Interviews Harry Potter - CollegeHumor Video

^ I notice she says si instead of shi... Personal quirk or some dialect? She's the cutest thing ever

Sounds to me more like deformed pronunciation caused by nervousness. In fact, I cannot catch too much of what she is saying without looking at the subs even though I am a native speaker. The girl appears to be from Gifu, where I have been many times but have not noticed the shi-to-si change among the general public.

松田桂奈 プロフィール - あのひと検索 SPYSEE [スパイシー]

It sounds more like a lisp than a dialectal issue.

You guys are both wrong. Harry just flicked his wand and whispered "accio mispronouncio" before confronting her.

And accidentally, the spell worked on Nissin as well.
______

or they are all Polish, as we pronouce si as shi :D

:D I didn't know you were Polish!

I've been to Gifu quite a few times myself and have never noticed that kind of pronunciation. Even if it existed wouldn't one avoid such an obvious regional accent like that when doing an interview?

To be honest, I haven't watched the interview... but as far as replacing si for shi goes, I have heard it done many times before. It's used as like a "cute" style of speaking. Like instead of うれしい　someone would say ウレセィー　(which I hope doesn't get confused with ウルせー haha). たのしい would be come タノセィー etc.

Is that along the lines of what is being talked about? I'd like to watch this video later on.

I'm not really familiar with harry potter and all that, and I think that just made that interview all the more hilarious to me. I loved the last thing that she said.

Going back to the topic at hand, I think what I originally thought was correct. Saying ワタセィ is just a cute-feminine way of speaking.

I think she was nervous, but I don't know if that affected her pronunciation or not. I've never seen that person before so I don't know how she normally sounds... so I guess I'm in no position to say for sure.

A tiny bit off topic, but...

Everyone I know writes it as スィ, not セィ. I would read スィ as si, but not セィ - which would just be a very short sei.