Does every kingdom have its own currency (its all silver, etc. but without the right face on the coin it won't be accepted legally and/or is worth less?
No. GP in my campaign are like Spanish pieces of eight in the real world c. 1600-1850. They are valid currency across a vast area. (Spanish "dollars" were legal tender from Hong Kong to Mexico to the US at the time.)
Does every region has its own dialect and every kingdom its own language?
No, I'm running a home version of Greyhawk. Most countries speak Common, the language of the old Great Kingdom.
Are serfs bound to the land they live on and property of their lord who can micromanage them as much as he wants (for example who they can/have to mary)?
Not in the countries the PC's have visited. Slavery is actually more common than serfdom in most of the campaign world.
Is literacy a sign of wealth or nobility as schools hardly exist and lower class children have to work anyway?
No. Everyone but barbarians is literate by default.
Is sanitation and hygene virtually unheard of, with only very big and wealthy cities having a sewer?
No and yes. Sanitation and hygiene exist. But only ancient, wealthy cities have a sewer. Most use cess pits and midden heaps.
Is the church extremely powerful and even the mightiest of kings bowing to their wishes?
Not in most countries.