The Avion My Uncle Flew by Cyrus Fisher.

A historical fiction, adventure, spy story.  Great boys book! 

I read this to my son ages ago and still recall how much we enjoyed it.  The main character, a boy named Johny Littlehorn,  is in France visiting an uncle and doesn't know any french.  As the story progresses he learns more and more words (and so do you). By the end of the book you can read some french. In fact the final 2 pages are all in the french that you've learned throughout the course of the story. We had a lot of fun doing accents. Unfortunately I know zero french so I had no idea if I was pronouncing things even close. Probably not, but it was alot of fun. And there is lots of history involved too. 

Here is what is on the back cover:

Johnny Littlehorn is on his way to France to live with his Uncle Paul. But what starts of as an exciting trip soon turns into a frightening adventure. A strange man trails Johnny throughout Paris and seems to follow him to his uncle's home in small St. Charmant. And then, as Johnny explores the town with his new friends, he comes to realize something even worse: A Nazi spy is hiding in the mountains, hatching an evil plan. Could the two men be one and the same? And can Johnny foil the villainous plot?

Enjoy!