As one of the men who framed Lavond is reading about his escape from prison, the paper he's holding is shown both folded and unfolded between shots.
Marcel explains to Lavond that he can reduce the size of atoms in a body, thus shrinking objects proportionally. Atoms are elemental particles and cannot be reduced in size.
The shrunken animals do not cast shadows when they move. This is obvious with the dogs on the lab table and the horse galloping on Radin's desk.
Like many American films set in France, the characters often drift between speaking English (sometimes with a French accent) and speaking French. In particular, many characters use "monsieur" and "mademoiselle" instead of "sir" and "young lady." Also, newspaper stories and wanted posters are seen in English but signs at the Eiffel Tower and elsewhere are in French.
Toto's cab has right-hand drive, but in France they drive on the right side of the road (as seen in the rear projection). It's possible the cab was made in England--but that's really a stretch.