AI in Japanese: Business Emails, Keigo, Translation

AI in Japanese is good enough for daily business use, with one condition: you have to tell the assistant who is writing to whom. Japanese grammar changes with the relationship between speaker and listener, so an assistant that does not know whether you are writing to a customer, a boss or a friend will guess, and the guess is sometimes too casual, sometimes absurdly stiff. Give it the relationship, the situation and the facts, and modern assistants produce natural business e-mails, summaries and translations that need light review rather than rewriting.

This guide is for people who work with Japanese customers, colleagues or suppliers, whether or not Japanese is their first language. It covers politeness levels, e-mail conventions, formats, translation traps, privacy rules in Japan and a few prompts you can adapt. It is not a language course, and anything with legal or reputational weight deserves a native reader.

How well does AI handle Japanese?

Japanese is widely represented in training data and current assistants write fluent, grammatical Japanese in all three scripts: kanji, hiragana and katakana. They handle compound words, counters, particles and the usual business phrases well. The weak points are the ones a non-native writer also struggles with: choosing the right politeness level, knowing which expression suits a particular company culture, and interpreting what an ambiguous sentence leaves unsaid.

Ask Mio works in 25 or more languages, including Japanese, and its own website is available in Japanese. You write in Japanese or in English, you never choose a model, and Mio routes the request to the best one for the task. The general approach to working in another language is in our multilingual AI assistant guide; this article adds the Japanese-specific layer.

Politeness levels: the thing to specify every time

Japanese has regular patterns for politeness, and business writing mixes them deliberately. At a basic level, there are three layers that matter in practice. See the overview of honorific speech in Japanese for the full picture.

Layer	Japanese name	Used for	Example
Polite	丁寧語 (teineigo)	Neutral courtesy: the baseline of business writing	確認します → 確認いたします
Respectful	尊敬語 (sonkeigo)	Raises the other person’s actions	見る → ご覧になる
Humble	謙譲語 (kenjōgo)	Lowers your own actions towards the other	送る → お送りする、お送りいたします
Plain	普通体 (futsūtai)	Friends, internal notes, some web copy	確認する

The classic error is mixing the directions: using the respectful form for your own action or the humble form for the customer’s. Assistants usually get this right when the relationship is clear and wrong when it is not. Put the relationship in the prompt: 取引先の部長 (a manager at a client company), 社内の同僚 (a colleague), 一般のお客様 (a consumer). A line such as “お客様向けの丁寧な敬語で、堅すぎず” sets the register.

Writing Japanese business e-mails

Business e-mail in Japan follows a recognisable shape. You can ask for it by name: “ビジネスメールの形式で”.

Subject line (件名). Specific and short, often with the topic first: 【ご確認のお願い】10月分の請求書について.
Addressee. Company, department, name and title, followed by 様: 株式会社〇〇営業部佐藤様.
Opening greeting. The standard phrase お世話になっております, for existing contacts.
Self-identification. A short introduction of who you are if it is a first contact.
Purpose and details. The request or information, in short paragraphs, with dates and deadlines explicit.
Closing request. Typically よろしくお願いいたします.
Signature. Company, department, name, contact details.

A prompt that works

取引先の営業部長宛てに、納期変更のお詫びとお願いのビジネスメールを書いてください。丁寧な敬語で、言い訳はせず、事実と新しい納期を明確に。事実:注文番号4821、元の納期10月23日、新しい納期10月28日、原因は部品の入荷遅れ。事実にないことは書かないでください。

Translated, the prompt asks for a business e-mail to a client’s sales director apologising for a delivery change, in polite keigo, without excuses and with the facts and the new date stated clearly, inventing nothing. The discipline is the same as in any language: facts supplied, additions forbidden, tone named. You can write the prompt itself in English and ask for Japanese output; the result is usually fine, but a prompt in Japanese tends to produce more natural rhythm, as discussed in non-English AI prompts.

Formats the assistant should follow

Item	Japanese convention	Typical AI slip
Date	2026年10月22日 (year-month-day)	10/22/2026 or October 22
Era years	令和8年 in official documents (2026 is Reiwa 8)	Using Western years where an era year is expected
Time	24-hour: 14:30 or 14時30分	2:30 pm
Currency	12,500円 or ¥12,500, no decimals for yen	12,500.00
Large numbers	Grouped by ten thousand: 万 (10,000), 億 (100,000,000)	Literal translation of “million” without conversion
Names	Family name first: 佐藤花子; honorific 様 or さん	Western order; first name with a title
Brackets and quotes	「」 for quotation, 【】 for emphasis in subjects	English quotation marks
Punctuation	Full-width: 、。	Half-width commas and periods mixed in
Tax and invoices	Tax-inclusive or exclusive stated: 税込, 税抜; invoice registration number where required	Silent omission

Put the rules that matter to you in a standing instruction and keep them in a project. A short example: “常に丁寧な日本語(です・ます調)。日付は2026年10月22日の形式、金額は12,500円(税込/税抜を明記)、句読点は全角。事実にないことは書かない。” Check any tax or invoice-related wording against current official guidance, because rules and rates change.

Translation between Japanese and English

Machine translation of Japanese has improved sharply, but the language still sets traps that explain most remaining errors.

Dropped subjects. Japanese often omits who is doing what. Translating into English forces a choice, and the assistant may pick the wrong person. In sensitive messages, ask it to list every place where it had to infer the subject.
Indirect refusals. 検討します or 難しいかもしれません can mean a polite no. A literal translation says “we will consider it” and misleads the reader. Ask for a note on the likely intention.
Politeness lost in English. Going from English to Japanese, a direct English request can sound blunt unless you ask for cushioning phrases such as お手数ですが and 恐れ入りますが.
Terminology and names. Company names, product names and legal terms should be fixed in a glossary, including the reading of names in kanji. Never let the assistant guess a person’s reading.
Katakana loanwords. English words in katakana may have shifted meaning; a glossary prevents surprises.

The review loop in AI translation quality: what to expect and how to improve it applies directly: supply context, a glossary and the audience, then check numbers, names and dates by hand. For contracts, regulatory text and anything with legal consequences, a qualified translator signs off; the assistant drafts.

Customer service and web copy in Japanese

Customers in Japan have high expectations for politeness and precision. An assistant can draft replies from facts and a policy, translate incoming messages so one agent handles several languages, and summarise long threads. Put the standard answers, the tone guide and product facts in a project so each reply starts from the same base, then have a native speaker read a sample before you go live. For web copy and product descriptions, ask for a specific style: an enthusiastic but polite tone for retail, a restrained one for professional services. Compare against a native competitor’s page to calibrate; an assistant matches the tone you name, not the tone you meant.

The same caution about sentences that “sound right” applies. Japanese readers notice slightly wrong keigo immediately, and a message that is grammatical but stiff or oddly casual can cost trust. Prefer a native reviewer for the first batch, and treat their corrections as additions to the project’s instructions.

Meeting notes, reports and long documents

Japanese organisations produce a lot of structured documents: 議事録 (meeting minutes), 報告書 (reports) and 稟議書 (approval requests). An assistant is good at turning rough notes or a transcript into minutes with the usual headings: date, attendees, agenda, decisions and action items with an owner and a deadline. Ask for 決定事項 and 今後の対応 as separate lists, and for each item to carry a name and a date. When the input is a long Japanese PDF, upload it on a plan that includes file uploads and ask questions in Japanese or English; request the page or section for every answer so you can verify it. The mechanics are described in how to use AI for meeting notes and summaries.

Learning Japanese with an assistant

For learners, an assistant is a patient tutor. Ask it to correct your sentences and explain each correction in English, to produce readings in hiragana for kanji you do not know, to write short dialogues at your level and to quiz you. Ask for furigana (readings above kanji) when you want to read and not only write. Keep in mind that correcting your Japanese is a different task from checking whether it is appropriate for a specific boss; for the latter, state the situation. More general guidance is in best AI for language learning.

Privacy and Japanese rules

Japan regulates personal information under the Act on the Protection of Personal Information (APPI), supervised by the Personal Information Protection Commission. Companies serving customers in Japan, or based there, should check with their own adviser what applies to sending personal data to external services. The practical habits are the same as everywhere: do not paste customer lists, names with contact details or ID numbers into prompts, keep only what the task needs, and use a vendor whose handling you can document. Ask Mio does not use chats or files to train models and lets users export or delete their data at any time. Our what not to share with AI list is a good checkpoint, and for an overview of the legal landscape for European customers see the EU AI Act explained in plain language.

Cost for a Japanese-language workflow

Point costs in Ask Mio are per answer, not per character, so Japanese text does not cost more than English text for the same request: a normal reply is 1 point, a long one 3, a coding answer 5 to 10 and an image 20. The free plan gives 600 points a month with Chat and Write modes. The Chat plan at €5 a month adds Research with web search, file uploads (PDF, DOCX, images), memory and 2,000 points, which is useful for uploading a Japanese contract or report and asking questions about it, and for keeping a project with your style rules. The Business plan at €29 a month has five seats, 15,000 points and shared projects. Prices are in EUR with 21% VAT added at checkout; the live pricing page is the reference.

A checklist before you send

I told the assistant who the reader is and the relationship.
Honorific direction is right: respectful for them, humble for us.
Names, titles and company names match the glossary.
Dates, times and amounts follow the Japanese format; tax status is stated.
No facts, deadlines or promises were added by the assistant.
A native speaker has seen the first messages of any new template.

Frequently Asked Questions

Does AI write natural Japanese?

For business e-mails, summaries and general translation, yes. Modern assistants write fluent, correct Japanese when given the relationship, situation and facts. Weak spots are the right politeness level, culture-specific phrasing and ambiguous input, so review the first messages of any new template with a native speaker.

How do I make the AI use the right keigo?

State who is writing to whom and name the register, for example a polite business tone for a client’s manager. Mention that respectful forms go to the other person’s actions and humble forms to yours. Then check the output for mixed directions, which are the most common error.

Should I prompt in Japanese or English?

Either works. An English prompt asking for Japanese output is fine for simple tasks, but a Japanese prompt tends to give more natural rhythm and tone. For important messages, write the facts in your strongest language and the style instructions in Japanese, then verify the result.

Can AI translate Japanese contracts?

It can produce a useful draft and a plain-language summary, but contracts and legal texts need a qualified translator or lawyer to confirm meaning. Keep official terms consistent with a glossary, check every figure and date against the original and never rely on a machine translation as the authoritative version.

Is it safe to use an AI assistant with Japanese customer data?

Only with care. Japan’s personal information law applies, so avoid pasting names with contact details, ID numbers or customer lists, keep to what the task needs and choose a vendor whose handling you can document. Check with your own adviser for your situation. This article is general information, not legal advice.

Which plan do I need?

Language support is the same on every plan. The free plan covers Chat and Write with 600 points a month; paid plans from €5 a month plus VAT add file uploads for Japanese documents, Research with web search, memory and larger point allowances. A team of up to five uses the Business plan.

The Bottom Line

AI in Japanese works for real business writing when you name the reader and the register, supply the facts, fix the formats and glossary in a standing instruction and keep a native speaker in the loop for the first batch and for anything important. The assistant removes the typing and most of the grammar risk; the relationship and the judgement stay with you. Try it on one real e-mail today, then compare the Ask Mio plans if you want document uploads and a project that remembers your style.

AI in Japanese: Business Emails, Politeness and Translation