I want to say this movie was an early 90s or perhaps late 80s color film in English.
It is about this boy, of about 10-12 years of age, who is staying in a castle with his mother and (I believe) her boyfriend. He meets and befriends this young girl who no one else can see. It turns out the girl is either a witch or a ghost, or something of a supernatural nature, and her mother comes to get her in the end, after leaving her alone in the castle there alone as a punishment.
I vividly recall one scene where the boy and girl go roller skating through the castle on connected roller skates, because the girl had never gone skating before. They are having a blast and then he skates into his mom, who thinks the boy has lost it because she cannot see the girl.