I am searching for a book I read around 10 years ago (in French).
The story was about a family going on vacation (I remember a sort of camping, near the sea). The protagonist was a young boy (8-12 years old) and his parents wanted him to learn a new language. He encounters a foreign girl who speaks Norwegian or Swedish (or a related language) and they become friends.
But instead of teaching French to the girl, he invents new words and begins to write a dictionary of this new language (so he won't forget the words he invents).
At the end of the book, the protagonist's sibling got lost and the girl saw him. So she tries to explain to the protagonist's parents in "French", but they obviously don't understand. But the protagonist understands and the sibling is found.
I also remember that there was a note from the author, saying that it was a true story inspired by his childhood. And that, growing up, he learned many languages, but still had to learn Swedish/Norwegian.