This Chingling has the "FRE" marker, despite not originating from a game set to a French region (image courtesy of Pika250)
The language icon glitch is a natural glitch in Pokémon X and Y. This glitch is an error in which a foreign language icon shows up on a Pokémon's summary from the same region as the player.
1. View the summary of a Pokémon from a different region to that of the player (specified by a language mark such as "FRE"); example video: Ledyba
2. Switch to a Pokémon from the same region as the player with no foreign language mark; example video: Chingling
3. Change to the second page of its summary
4. Switch back to the original foreign region Pokémon while still on the second page (example video: Ledyba)
5. Switch back to the same region Pokémon while still on the second page (example video: Chingling)
6. Switch to the first page, and the same region Pokémon will now erroneously have the foreign region mark from the other Pokémon