九州、四国、本州に囲まれ、１１の海岸線を持つって英語でなんて言うの？

囲まれる：to be surrounded by
海岸線：coastline(s)
瀬戸内：Setouchi (Inland Sea)
内海：inland sea

The Setouchi Inland Sea is surrounded by Kyushu, Shikoku, and Honshu, and has 11 coastlines.