This gift was given to players that visited any of the Pokémon Centers in Japan during their birthday month.
It should be noted that regardless of the player's game save language, this Chansey will be received as "JPN" (will have "JPN" language tag on non-Japanese languages).
However, the nickname is set to default to the player's game save language.
(so for example, if you're playing on English, you'll get "Chansey" as the name, and it'll bear the JPN language tag, as seen in the screenshots section)
This however causes issues if you're playing on Korean: the Chansey will have "??" as the name (even though the data of the file shows it was assigned the correct name).
It appears to be a glitch caused by the game: Any Pokémon will JPN language tag cannot have a Korean name, as seen in the screenshots)
This can occur naturally, by simply trading a JPN caught Pokémon to a KOR game and evolving it there.
Format Ver.1.0.7-0, Post Updated Date:20190425_0656