He continued speaking in Japanese even if they didn't understand him.
「通じているかいないかは関係なく、彼は日本語で喋り続けた」はこのようにも言います：
He continued speaking in Japanese even if they didn't understand him.
continued speaking in Japaneseは「日本語で話し続けた」という意味です。
even if they didn't understand himというのは「彼（の言葉）をわかっていな買った場合でも」です。
もしもっと詳しくに言うと、HeをMy sonに、theyをhis foreign friendsに入れ替えてもＯＫです。
My son continued speaking in Japanese even if his foreign friends didn't understand him.
He kept on speaking Japanese to non-Japanese friend in his neighborhood.
He didn't mind if his non-Japanese friend in his neighborhood didn't understand what
he was speaking and kept on speaking Japanese to the friend.
「彼は、近所の外国人の友達が話を理解しているかどうか気にせずに、日本語を話し続けた。」
ご参考になれば幸いです。
アメリカ合衆国 
日本 
