My girlfriend’s family is very formal unlike mine. I mean, my family’s formal too but not as formal as my girlfriend’s. They are Christians like us, however, their practices are killing me. Like, they encourage me to wear something at least presentable (polo and jeans), to attend in every church activities and to go to church every Sunday. I am a foreign man myself and they are expecting me to do all those. How should I handle this kind situation? It’s kinda hard.

Your clothes are not just elements that you wear to cover your body, they are an extensión of your personality and they are supposed to talk about who you are.
If this girl is important for you it's natural that you want to cause a good impression to her parents, after all they are your girlfriend's family; however, find a flexible point where you feel you are still yourself and you feel happy with the clothes that you wear; this way you can show to her family that you are making an effort to cause them a good impression.
If you cross the line and you try to please them totally and you forget about yourself you'll fall down into a trap, and you will feel frustrated later.
I´m also Christian, and I agree that it's necessary to show respect when you go to the church, and dress properly, however, your fate has nothing to do with the clothes that you wear as long as you don't dress been exhibitionist and disrespectful but I think that's not your case.
Do the necessary to gain the respect from her family, but don't lose yourself; If you are the right person for this lady then your actions will speak louder than your clothes or your words and her family will get to see that.