おじいちゃんって英語でなんて言うの？

If you actually call your father or mother's father 'Grandfather' then it seems quite formal. Most common would be Grandpa or Grandad.
Eg "We are all going to visit grandad this weekend."

Grandfather is a formal way of saying 'father of my parent'.
A casual way of saying it would be either 'Grandad'. In American English, it is spelled 'Granddad', but in British English it is spelled 'Grandad'.
A very easy way to say it is 'Grampa' or even 'Gramps'. These two are commonly used by very young children.

"Granddad" or "grandpa" are informal ways to refer to the father of one of your parents.
"My granddad is almost 75 years old."

'grandad'', ''grandaddy'', ''grandpop'', ''grand-daddy'' are informal terms used to refer to a grandfather.

"paternal" and "maternal" are good words to use if it's important whether the grandparent is your mother's parent or your father's parent. These words often appear if you are talking or reading about a person's ancestry or DNA. Americans whose families came from another country (that is, almost everybody) can use it if they are trying to talk precisely about their family origins. However, paternal/maternal grandmother/grandfather are formal-sounding words, so, for example, you wouldn't use it if you are introducing your grandparents to someone. In normal conversation, you might just say, for example, "Here is my grandfather", even though it's not immediately clear which grandfather he is.

Grandfather' is often the most formal way of referring to the father of one's parent.

We can use 'grandad' and 'grandpa' to be more casual. Many people have their own variety of ways to refer to their grandparents such as 'gramps' and 'pops', amongst others.

Grandfather is more formal, but "Grandpa" and "granddad" are more casual ways of referring to your father's father. For example; you can say:
-I am visiting my paternal granddad for Christmas this year because I haven't seen him in two years.
-My maternal grandpa is the best. He always gets me the best gifts for my birthday.

Depeding on what country you come from would decide what term you use to describe your mother or fathers father
In the UK 'grandfather' or 'grandad' would be more common to say

Here the most commonly used words to describe the father of one's parent would be to say the above "Grandfather" "Grandad" or "Grandpa".
Here are some examples -
"I'm going to see my grandfather today."
"Myself and my granddad are going to the shopping centre today".

Grandad is the most common name for your mother and father's father.
Grandpa is a cuter way of saying Grandad.