There is a saying in Japan that if you are not good to your parents, you get a hangnail.
ご質問ありがとうございます。
"There is a saying in Japan that if you are not good to your parents,"＝「日本での言い伝えには、もしあなたが親孝行でなければ、」
（"if you are not filial to your parents"＝「もしあなたが親孝行でなければ」という言い方もできます。）
"you get a hangnail."＝「ささくれが出来ると言われています。」
☆「迷信」は "superstition" です。
ご参考に。
There's a Japanese superstition that says kids get a hangnail if they are disobedient to their parents.
Japanese people believe that they may get a hangnail if they are undutiful towards their parents.
この場合、次のような言い方ができます。
ーThere's a Japanese superstition that says people get a hangnail if they are disobedient to their parents.
「親不孝だとささくれができるという日本の迷信があります」
ーJapanese people believe that they may get a hangnail if they are undutiful towards their parents.
「日本人は親不孝だとささくれができると信じています」
superstition「迷信」
to get a hangnail「ささくれができる」
disobedient to one's parents「親不孝な・親に不従順な」
undutiful towards one's parents で「親不孝な」とも言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
