If you're too proper when you speak English with a friend, I think there will always be a wall between you and your friend.
If you don't learn to speak casually with new friends you have made, you put up a barrier between you and them.
この場合、次のように言うと良いと思います。
ーIf you're too proper when you speak English with a friend, I think there will always be a wall between you and your friend.
「友達と礼儀正しすぎる英語を話してると、いつまでも友達との間に壁があると思う」
proper で「礼儀正しい・正確な」
be a wall between A and B で「AとBの間に壁がある」
ーIf you don't learn to speak casually with new friends you have made, you put up a barrier between you and them.
「新しく作った友達とカジュアルに話さないと、友達との間に壁を作るよ」
to learn to speak casually で「カジュアルに話すことを学ぶ」
to put up a barrier で「壁を作る」
ご参考まで！
カナダ 
