The Avion My Uncle Flew, by Cyrus Fisher

Johnny Littlehorn lives on his family's ranch in Wyoming during World War II.  Because his father is away, Johnny decides to help bring in the cattle one night.  Unfortunately, he sustains a serious leg injury on his horse.  He has it treated, but will need to see a specialist.  While trying to decide where they should travel to (New York City or San Francisco), his father unexpectedly returns.

Mr. Littlehorn was injured in the war, and as he was not fit to rejoin his unit, he was given a position as a liaison in Paris between the French and British governments.  The war has ended but he is still needed there; as his wife is French, Mr. Littlehorn proposes that the entire family relocate to France so they can be closer together, and so Johnny can get to know his uncle Paul, who still lives near the family's ancestral home.  As an added bonus, one of the specialists who would be qualified to treat Johnny's leg is in Paris, sealing the deal.

Once in Paris, the family befriends a porter, Albert, from the hotel.  He agrees to regularly wheel Johnny around the city for fresh air.  But during these excursions, when he's alone while Albert runs an errand a strange man keeps making conversation with Johnny in the park, asking pointed questions about his family and their home in St. Chamant.  The man seems suspicious and makes Johnny uncomfortable, but he can't communicate with Albert and his parents don't take him seriously.  Eventually, the man tries his utmost to convince Johnny not to go to St. Chamant to see his uncle Paul, but Johnny has little choice in where he goes.  Mr. Littlehorn is recalled to London, and Johnny's parents decide that to aid his recovery, Johnny should go to St. Chamant to stay with Paul.  To make it appealing, they promise him that during his three-month visit, if he is able to teach himself to walk again and learns enough French to write his mother a letter, he'll receive a bicycle with gears and a dynamo-powered light on the front (an item almost nonexistent in Wyoming, sure to be the envy of all of his peers).

Uncle Paul comes to Paris to retrieve him, and is a kind and friendly fellow.  Although poor, he is generous with Johnny, and confides his plans: he is attempting to invent and build a new type of glider plane so he can sell it to a bigger company.  With the proceeds, he will rebuild the family home, which was destroyed by Germans as they left the country following the occupation.  As they are traveling via train to the village, however, they see the suspicious man from the park.  Johnny is convinced that they are being followed.  Uncle Paul agrees to write to his friends in the police in Paris, but suspects that the man is an agent of the mayor of St. Chamant, who has made it clear that he'd like the property for himself.

Over the Summer, Johnny (now called Jean) becomes a part of the community and makes a few friends his own age, gradually learning the language.  But he still looks over his shoulder for Albert and the man from the park.  And his anxiety really kicks into high gear when he finds a German soldier's recently-abandoned back pack in the ruins of the family home, with a pistol inside!

Suffice it to say, this is a fun adventure-y sort of book for middle grade readers.  Additionally, the reader learns French vocabulary along with Jean (so some background in the language would help if for no other reason than to know how to pronounce it), and by the end of the book, if you've paid attention, you've got enough vocabulary to read the letter he writes to his mother.

Some interesting notes on the author (who wrote this book under a pseudonym) are here: http://www.enotes.com/topics/darwin-l-teilhet  A midwest resident like his protagonist, he also spent time in France as a teen.  He also worked for US Intelligence in Britain during the War, and wrote mystery novels and spy thrillers.  This book is a worthy extension of his talents!