rudeは、人に対しても使いますが、「マナー違反」という意味でもよく使われます。
That's rude.で、「それはマナー違反ですよ」となります。
「～したほうがいいですよ」と相手に促す時は、shouldを使いましょう。
must や had betterは強すぎるので、偉そうな感じになってしまいます。
相手がよほど親しい人（恋人や家族）でない限りは、使わない方が無難です。
That's considered rude in Japan, so maybe you should be careful about that.
That's actually rude here in Japan, so you should probably be careful about that.
People would be offended by that here in Japan, so you should take care.
英訳１：ここで使われている〈considered + 形容詞〉の形は、「…と見なさせる」という意味になります。
なお、maybe you should...という言い方もおすすめです。単にYou should...というよりソフトな口調になるので、注意ではなくアドバイスとして伝わります。
英訳２：actuallyで、「（それって）実は…」というニュアンスを表現することができます。
英訳３：be offended by ...は「…で気分を害する／腹を立てる」という意味です。
その他の表現：
- Japanese people find that offensive, so maybe you should be careful with that.
Please remember that in Japan that is considered very rude.
That is not something that would go down very well in Japan.
You can point out that some things are not acceptable in Japan as they may be in other countries. You could identify the phrase, word or activity and explain exactly what is not acceptable. 'To go down well' means a phrase or activity that would be welcomed.
他国で受け入れられていても、日本では受け入れられないこともあることを指摘するフレーズです。フレーズ、単語、またはアクティビティをいって、受け入れられないものを正確に説明することができます。 「go down well」とは、歓迎される言葉や活動を意味します。
The first two examples are more or less the same with a movement of, "be careful," either to the beginning or end of the sentence. The last example is using the phrase, "to be considered," which talks about the perspective of whoever is the object of the sentence.
I would be careful of saying/doing that because it is considered rude in Japan.
In Japan, people consider that impolite. So, I would avoid saying/doing that.
If you say/do that, you may offend someone because that is considered rude in Japan.
You can use words like "be careful of" or "avoid" to help the person understand that they should refrain from doing/saying something that is offensive. To explain why, you can say, "it is considered rude in Japan", "people consider it impolite" or "someone may be offended".
「気をつけてください」や「回避する」のような言葉を使い、失礼なことをやらないように、言わないように伝えることができます。。 理由を説明するには、 "it is considered rude in Japan", "people consider it impolite" や "someone may be offended". と言うことができます。
Be careful, that is considered rude here in Japan.
Be mindful of your actions whilst being here, it might be considered rude!
Don't do that, we might still be respectful towards you, but we actually think it is very rude.
With the above statements you can help your friend when he/she is doing something culturally incorrect in Japan. After stating that what they are doing is incorrect, you could maybe help them to correct their behavior. You could say "Be careful, that is considered rude here in Japan. Rather do..."and then you could show them. You can use the word "mindful" instead of careful. This indicates that you would like them to think before they take action. Hope this was helpful!
example
"You better not do that, it's a bit of a faux pas in Japan"
or
"It's generally not acceptable behaviour in Japan"
or
"It's considered an indiscretion in Japanese culture"
You could also use words like
blunder,gaffe, or more formally
a breach of ettiquette
"You better not do that, it's a bit of a faux pas in Japan"
日本では、少し失礼になるからそれはしないほうがいいですよ。
"It's generally not acceptable behaviour in Japan"
それは、日本では一般的に受け入れられない行為です。
"It's considered an indiscretion in Japanese culture"
それは、日本の文化では、思慮に欠けるとみなされます。
また、 blunder（失敗をする）,gaffe（失言、失態）, さらにかしこまった言い方はa breach of ettiquette（無作法）という言葉も使うことができます。
That is seen as disrespectful in Japanese culture.
Avoid doing that, it is impolite in Japanese culture.
Please don't do that, it is rude in Japanese culture.
You can use these phrases :
That is seen as disrespectful in Japanese culture.
Avoid doing that, it is impolite in Japanese culture.
Please don't do that, it is rude in Japanese culture.
Japanese won't approve of that.
Please don't do that, Japanese doesn't like it.
It is rude to do that in Japan.
In Japan that is considered to be very rude so just be careful when saying/doing that.
Japanese culture finds that to be rude so just be careful.
if you would like to explain to someone that something in japan comes off very rude and for them to be careful, you can say something like "In Japan that is considered to be very rude so just be careful when saying/doing that." By using the word "careful" in our sentence, this tell the other person to be cautious when doing/saying something.
In Japan that is considered rude, make sure you don't do that in Japan.
In different countries there are different things that are seen as offensive, for example: the way you eat food, hand gestures etc. It's important to warn people about these things, so they don't offend anyone accidentally.
When introduced to a new country and culture, it is always important to understand their customs, traditions and culture. In some cultures doing or saying certain things may be seen as disrespectful or rude.
Examples of sentences using these words;
- It is offensive not to shake someone's hand when first meeting them.
- It is considered to be impolite when you don't greet when entering a room.
- It is disrespectful not to say goodbye before leaving someone's house.
日本 
アメリカ合衆国 
イギリス 
南アフリカ 
ニュージーランド 
