They both have a very similar meaning; however one difference is that interior can be used as a noun or an adjective, but internal can only be used as an adjective.
The building's interior. (The inside of the building - a noun, a thing.)
However internal can only function as an adjective.
The building's internal. (incorrect - internal is not a thing/noun, only a feature/description of a thing.)
The building's internal space. (correct - here internal (an adjective) describes the space as being internal, not external.)
There is much more to be said on this topic.
Hopefully someone can elaborate on my suggestions.
I am not a teacher.