He/She has lived abroad for so long but still speaks beautiful Japanese.
His/Her Japanese is perfect even though he's/she's lived abroad for so long.
色々な言い方があると思いますが、二つ考えてみました。
参考にしてみてください(*^▽^*)
He/She has lived abroad for so long but still speaks beautiful Japanese.
（彼／彼女は海外に長く住んでいたのに、美しい日本語が話せます）
His/Her Japanese is perfect even though he's/she's lived abroad for so long.
（彼／彼女は長い間海外に住んでいたのに、完璧な日本語が話せます）
日本 
