I would like to ask the difference between a flat and appartment and translation into Spanish.
In AmE, there is a distinct difference between a "flat" and an "apartment." A flat usually refers to a house that has been divided horizontally into two separate living spaces (one upper, one lower), each of which are fully self-contained with a kitchen, a bathroom and a bedroom and both of which have separate, private entrances.
An apartment is traditionally a smaller self-contained dwelling located in a larger complex. Some apartment buildings might have as few as four units, others might have 50 or more.
"Apartamento" is the Spanish word for "apartment," and a quick Google search of this word implies that it basically means the same as an apartment in the US - a rental place in which to live.
We don't have any "flats" for rent in my part of America, regardless of whether they were built as separate units, or larger homes later subdivided.