Not necessarily. Some versions in the Bohème line carry “France” alongside the serial number in place of the usual “Germany”. The reason is that individual components, sleeves and barrels in particular, were made by Cartier in France according to the manufacturer; pieces marked accordingly therefore left the factory that way. For the Bohème Je t'aime in precious resin with its heart-shaped ruby, collectors regard the same explanation as plausible, though it has not been established conclusively.
The country marking on its own is thus no proof either way, of authenticity or of forgery. More telling are the finish of the clip and clip ring, the execution of the star, a serial number struck cleanly and evenly, the engraving and grind of the nib, the faultless working of the retractable nib mechanism, and the fit and weight of the fittings. Anyone who wants certainty should have the pen examined by the manufacturer, who can also say which marking was intended for a particular version.