31. Interpreter/Liasion Officer
The silver tapes on cuff denote that this man is adjutant, and the uniform is an Interprète Militaire de Liaison. A trainee interpreter would be a corporal and the minimum rank for a fully qualified one would be sergeant, IIRC.
They would be assigned to the British staff at various levels (e.g. as part of a regimental HQ) for communication with both the French military and the locals (e.g. arranging food purchasing, accomodation of officers/soldiers and stabling of horses etc.).
The uniform is British other than the hat, which has the rank braid around the top. On the blue collar tab which denotes a particular branch, is a metal Sphinx badge, signifying interpreter.