We Japanese learned unnatural English at school which is why our English is so strange!
We were taught unnatural English when we were at school. That's why our English is so strange!
Japanese people's English is unnatural because that's what we were taught at school!
「学校でおかしな英語を習ってきたから、私たち日本人の英語は
みんなヘンテコなんですよ!」= We Japanese learned unnatural English at school which is why our English is so strange! / We were taught unnatural English when we were at school. That's why our English is so strange! / Japanese people's English is unnatural because that's what we were taught at school!
ボキャブラリー
we Japanese = 我々日本人
learned = 学んだ、覚えた
unnatural = 不自然な
why = だから、という理由で
so strange = そんなに変、とても変
were taught = 教えられた