In Japan, washing before getting into the bathtub is a cast-iron rule.
Washing thoroughly before taking a bath is a non-negotiable rule in Japan
In Japan, removing your shoes before entering the house is a cast-iron rule.
「鉄の掟」＝ a cast-iron rule / non-negotiable rule
「湯船に浸かる前に体を洗うのは日本では鉄の掟です。」＝ In Japan, washing before getting into the bathtub is a cast-iron rule. / Washing thoroughly before taking a bath is a non-negotiable rule in Japan
「玄関で靴を脱ぐのは日本では鉄の掟です」＝ In Japan, removing your shoes before entering the house is a cast-iron rule. / It is a cast-iron rule in Japan to take your shoes off before entering a house
In Japan, washing yourself before getting into the tub is practically a hard-and-fast rule.
こんにちは！
ご質問ありがとうございます。
ご質問について、いくつか言い方が考えられますが、例えば、
In Japan, washing yourself before getting into the tub is practically a hard-and-fast rule.
とすると、「日本では湯船に入る前に体を洗うのは、ほとんど鉄の[掟](https://eikaiwa.dmm.com/uknow/questions/5276/)のようなものです。」となります。
役に立ちそうな単語とフレーズ
hard-and-fast rule 絶対的な決まり
practically ほとんど〜のようなもの
参考になれば幸いです。
イギリス 
日本 
