First, the term "wheelchair" is mostly 20th C., referring to something with big back wheels the person in it could move around. Before then, references are to Bath chairs or invalid chairs which have small wheels and must be pushed by somebody else. You can see this in places like the Sears catalogs. I only pursued this a while out of idle curiousity, but I found British references into the 1700's. So looking under the other term will get you back farther. The question then becomes in which country & therefore language it developed. I'm betting on France or Italy.