I would actually love to do that and return with my accent not changed one little bit just to prove a point.
And even if my accent was slightly changed I sure as hell wouldn't be acting like I don't understand English and legit acting like a foreigner.
You probably have never been around people who don't fully understand a language, they have a completely different demeanor, they very often smile alot because they don't understand what is being said completely and they use their hands for expression, these are traits that someone has when they don not understand a language..
Are you still with me? I'm trying to explain it so the simplest minds can fathom it.
So when someone who speaks the language because they grew up there and have been video taped speaking it perfectly that person would not have these same characteristics as someone who has never spoke the language.
A little twang to your accent maybe but it will never completely change, ever unless you want it to.
Let me know if you need anymore help, sweetheart