You're going to have to take off your swimsuit. It's Japanese bath etiquette.
Wait, you have to go in naked. Sorry, but those are the rules here.
I know you're not used to it, but going in naked is the norm here. Trust me, it's not that bad.
お風呂周りの文化って結構違いますもんね、日本と海外で！
グッドマンさんの仰る通り、
「裸で入らないといけない！」
よりも、
「水着を着たまま入らないでね！」
と言った方が、一般的に自然な印象になります。
追加の言い方として、以下があります：
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You're going to have to take off your swimsuit. It's Japanese bath etiquette.（「水着を脱がないとダメだよ。これは日本独特の、お風呂のルールだね。」）
Wait, you have to go in naked. Sorry, but those are the rules here.（「悪いけど、服を脱いでから入ってね。日本の文化だよ。」）
I know you're not used to it, but going in naked is the norm here. Trust me, it's not that bad.（「ちょっと引くかもしれないけど、日本では（お風呂に）こうやって裸で入るのが当たり前なんだよね。意外とすぐ慣れるから、心配しないで。」）
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僕個人としては、日常的なことにおいても、笑いを取れそうな場合はガンガン狙いちゃいたいので、
相手がちょっぴりびっくりしても全然大丈夫！（むしろ、その方がいい）というスタンスですね。
なので、外国からやってきた友人が、服・水着などを着たままお風呂に入ろうとすると、
以下で止めてみてください。
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Whoa, man. This is Japan. Please get naked.（「ちょっと待った！ここは日本。裸になれ。」）
Take it off. All of it.（「脱ぎな。全部。」）
Get naked now.（「裸になれ、今すぐだ。」）
Take off your clothes or I'll have you thrown out.（「裸にならないと言いつけるよ？」）
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It may come as a bit of a shock for some tourists to hear this. In the UK this is not the case. Many UK tourists would probably choose not to bathe at all if that is the house rule. In the UK many people are shy or simply not used to bathing naked in front of other people.
You have to be stark naked when going into a public bath in Japan. No swimsuits are allowed.
In Japan, you have to be completely naked when going into a public bath. It is the general rule.
The phrase 'stark naked' used in the first statement means 'completely naked', without anything like a swimsuit. This is very similar to what you would do when getting into a 'nudist colony' in countries where they exist.
So, you may say:
You have to be stark naked when going into a public bath in Japan. No swimsuits are allowed.
or
In Japan, you have to be completely naked when going into a public bath. It is the general rule.
stark naked'という言葉は 'completely naked'と同じで水着なども無しの真っ裸という意味です。ヌードコミュニティ'nudist colony' の人々と同じような状態です。
ですのでこのように説明できます。
You have to be stark naked when going into a public bath in Japan. No swimsuits are allowed.
（日本の公衆浴場に入る際は真っ裸にならないといけません。水着着用は認められていません。）
In Japan, you have to be completely naked when going into a public bath. It is the general rule.
（日本では、公衆浴場に入る時は完全に裸にならないといけません。それが一般的なきまりなんです。）
We don't wear a swimsuit in public baths in Japan.
No clothing required for bathing here.
This will come as a surprise perhaps to foreign visitors who may be unused to being naked in front of strangers or naked in a public setting.
As a traveller though, they must get used to following etiquette in different countries. You are doing them a favour by telling them beforehand so that they do not feel even more awkward later on, when they realise they are not following the rules that are set.
When people travel, they should not want to offend other peoples' cultural beliefs and traditions, even if it is very different to their own.
It's the norm that you be naked when going into a public bath.
No one goes into the public bath with clothes on.
It is expected that you go inside a pblic bath with no clother on.
Tourusts who visit another country with different cultural norms amy not always know what is expected of them. If you want to explain to tourists from overseas that you have to be naked when going inside a public bath you can't wear a swimsuit, here are a few ways you can say that:-
1. It's the norm that you be naked when going into a public bath.
大浴場に入るときは裸になるのが一般的です。
A "norm" is something that is normal or expected.
2. No one goes into the public bath with clothes on.
服を着た人は誰も大浴場に入らない。
3. It is expected that you go inside a pblic bath with no clother on.
クローザーなしでpblic風呂に入ることが期待されます。
海外から来た人の中にはその国の文化についてよく知らない人もいると思います。
「銭湯には裸でないと入れない／水着を着用することはできない」と説明したいなら、次のように言えます。
1. It's the norm that you be naked when going into a public bath.（銭湯では裸になるのが一般的です）
。
"norm" は「標準的なこと」や「一般的なこと」を指します。
2. No one goes into the public bath with clothes on.（服を着たまま銭湯に入る人はいない）
3. It is expected that you go inside a public bath with no clothes on.（銭湯には裸で入るのが普通）
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