In Pokemon Emerald, contains a programmed event encounter with the Mythical Pokemon Mew.
However, the key item that activates the event, Old Sea Map, was never released to players outside of Japan.
This page hosts the Mew that is caught at Faraway Island.
As an extra illegal bonus, it's caught with the Safari Ball, that cannot be obtained outside of the Safari Zone.
It is possible to reach Faraway Island in Non-English and Non-Japanese other European languaged games,
using an elaborate usage of glitches, as explained by ChickasaurusGL below.
It is unclear whether the non-Japanese Mew can be sent up via Pokemon Bank.