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）
That guy lived overseas for a long time, but his Japanese is still very polished.
この場合、次のような言い方ができますよ。
ーThat guy lived overseas for a long time, but his Japanese is still very polished.
「あの男性は海外生活が長いのに、日本語がとてもきれいです」
ここの日本語がきれいなことは polished を使って表現することもできます。
ご参考まで！
日本 
カナダ 
