【教えて下さい】「18年間行方不明だった彼の弟が見つかった。」の英訳

foundですね。

has discoveredだと何かを発見したことになってしまうので、
His brother who was missing for 18 years was found.
が正しいです。

His brothre who had been missing for 18years has discoverd.

His brother, who was missing for 18 years, was found.

「18年間行方不明だった彼の弟が見つかった」
His missing younger brother was found for 18 years

http://ejje.weblio.jp/sentence/content/18%E5%B9%B4%E9%96%93%E8%A1%8C%E6%96%B9%E4%B8%8D%E6%98%8E%E3%81%A0%E3%81%A3%E3%81%9F%E5%BD%BC%E3%81%AE%E5%BC%9F%E3%81%8C%E8%A6%8B%E3%81%A4%E3%81%8B%E3%81%A3%E3%81%9F