Translate a Flutter app into Japanese

What Japanese changes in an ARB file

Japanese has a single CLDR plural category other. A message written for English such as {count, plural, =1{1 message} other{{count} messages}} therefore needs at least an other branch, for example {count, plural, other{{count} 件のメッセージ}}. Manuce checks the structure of plural and select messages and adds the target language's plural categories. An explicit =0 branch is still worth keeping if you want メッセージはありません rather than "0 件".

Japanese counts with counter words (助数詞) 件 for messages or results, 個 for objects, 人 for people, 枚 for photos or tickets. The right one depends on what {count} refers to, and the description in the @ key is what tells the translator.

Word order changes too, with the verb at the end Delete {fileName}? becomes {fileName}を削除しますか？. The placeholder moves, which ICU allows without touching your Dart code. A sentence assembled by concatenation in code ('Delete ' + name), however, can never read correctly in Japanese merge it into one message with a placeholder before translation.

Gender select messages ({gender, select, male{…} female{…} other{…}}) often end up with identical branches in Japanese, since most interface sentences do not mark gender. That is expected, not a fault.

Register, scripts and punctuation on screen

A Japanese interface uses the polite form (です／ます) in sentences and short noun forms on buttons 保存 for "Save", キャンセル for "Cancel". Error messages stay courteous 接続できませんでした。もう一度お試しください。 For apps aimed at business customers, more formal keigo (ご利用いただけます) is expected in onboarding text.

Three scripts mix kanji for concepts, hiragana for grammar, katakana for loanwords (アカウント, ログイン). Katakana loanwords can be long and every character is full-width, so a label like アカウント設定 may overflow a button sized for "Settings". Plan for TextOverflow.ellipsis or wrapping in narrow areas.

Punctuation is full-width 、 and 。, ？ for questions, and 「」 instead of straight or curly quotes. No spaces between words or before punctuation, and no full stop on buttons or headings.

A Flutter-specific trap kanji share Unicode code points with Chinese characters. If the app does not declare Locale('ja') in supportedLocales, or the font falls back to a Chinese design, some characters render with Chinese glyph shapes, which a Japanese reader notices at once. Declare the locale and, if needed, bundle a Japanese font such as Noto Sans JP.

What Manuce returns, and what it does not do

You upload your template file, say app_en.arb, in whatever source language you work in, and get back the same file with only the values in Japanese. Indentation, key order, quotes, comments, line endings, encoding and BOM are preserved. @@locale is switched to ja save the file as app_ja.arb in your translations folder.

@ keys are skipped their descriptions are used as context, and metadata such as placeholder types and date format stays untouched. Every translated value is compared with the source if a {count} or a tag is missing and the mismatch persists, the source value is kept and the report says so, so flutter gen-l10n does not fail.

Not translated keys, empty values, URLs, email addresses, numbers, colours, date formats, identifiers and anything you mark as non-translatable. You pay for the words in the values only, keys and tags excluded, with Japanese carrying a ×1.6 coefficient.

• Only the uploaded file is translated strings hard-coded in Dart stay in the source language.
• Rendering on a device (overflow, glyphs) is not tested.
• Manuce does not provide sworn or certified translation if Japanese terms of use must be certified, you need a sworn translator.

Preparing the project and checking afterwards

A little preparation in the template file prevents most mistranslations into Japanese

• Add a description to every ambiguous key "button that sends the form" steers Submit towards 送信 rather than 提出 (handing in a document).
• State what each {count} counts, so the right counter word is used.
• Keep dates and numbers out of the strings a DateTime placeholder with "format": "yMMMd" renders as 2025年3月14日 through intl.
• Add ja to your supported locales and, for iOS, to CFBundleLocalizations in Info.plist.
• Run flutter gen-l10n, then switch a device to Japanese look for truncated buttons, awkward line breaks and Chinese-style kanji.
• Read the report every string left in the source language is listed with the reason.

In practice

1. Add descriptions to the @ keys of your template .arb file, especially for counters and buttons.
2. Upload the file to Manuce and review the quote and the free Japanese sample.
3. Download the translated file and the report of strings left unchanged.
4. Save it as app_ja.arb, declare the ja locale and run flutter gen-l10n.
5. Check the layout on a device set to Japanese.