# Epub Translator

> Translates EPUB ebook files between languages with parallel processing. Supports Japanese, English, Chinese, and other languages. Handles large files by splitting into sections, manages multiple volumes simultaneously, and preserves EPUB structure and formatting. Includes translation quality validation. Use when translating novels, books, or any EPUB content.

- Skill: `nebu1eto/epub-translator` (Agent Skill, multi-file: 24 files)
- Install (CLI): `npx skillmds@latest add nebu1eto/epub-translator`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nebu1eto/epub-translator/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: Nebu1eto (https://skillmd.com/u/nebu1eto)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nebu1eto/epub-translator

---


# EPUB Translation Skill

Translate EPUB files between any language pair with optimized support for Japanese and English to Korean.

## When to Use This Skill

Use this skill when:
- User wants to translate an EPUB ebook to another language
- User mentions translating Japanese/English/Chinese novels or books
- User has multiple EPUB files to translate in batch
- User needs to preserve EPUB formatting and structure during translation

## Usage

```bash
/epub-translator <epub_path> [options]
```

### Arguments
- `<epub_path>`: EPUB file or directory containing EPUBs

### Options
| Option | Description | Default |
|--------|-------------|---------|
| `--source-lang` | Source language code | `ja` |
| `--target-lang` | Target language code | `ko` |
| `--dict` | Custom dictionary (JSON) | none |
| `--output-dir` | Output directory | `./translated` |
| `--parallel` | Concurrent agents | `5` |
| `--split-threshold` | File size for splitting (KB) | `30` |
| `--split-parts` | Parts to split large files | `4` |
| `--high-quality` | Prefer the runtime's stronger model for translation | `false` |
| `--vertical` | Output vertical writing (ja/zh only) | `false` |

### Language Codes
`ja` (Japanese), `en` (English), `ko` (Korean), `zh` (Chinese), `es` (Spanish), `fr` (French), `de` (German), `ru` (Russian), `ar` (Arabic), or any ISO 639-1 code.

## Examples

```bash
# Japanese novel to Korean (default)
/epub-translator "/books/novel.epub"

# English to Korean
/epub-translator "/books/english.epub" --source-lang en

# Japanese to English
/epub-translator "/books/jp_novel.epub" --source-lang ja --target-lang en

# High-quality translation using the runtime's stronger model
/epub-translator "/books/important.epub" --high-quality

# Batch with larger split threshold (less splitting)
/epub-translator "/books/" --split-threshold 50 --parallel 10

# More aggressive splitting for slower connections
/epub-translator "/books/large.epub" --split-threshold 20 --split-parts 6

# English to Japanese with vertical writing (우종서/縦書き)
/epub-translator "/books/novel.epub" --source-lang en --target-lang ja --vertical

# Korean to Chinese with vertical writing
/epub-translator "/books/korean.epub" --source-lang ko --target-lang zh --vertical
```

---

## Architecture

```mermaid
graph TB
    O["ORCHESTRATOR<br/>• Analyzes EPUBs and creates task manifest<br/>• Dispatches translation work using available agent/runtime features<br/>• Collects translated section files<br/>• Validates translation quality<br/>• Handles retries and error recovery"]
    T1["Translator<br/>Agent 1"]
    T2["Translator<br/>Agent 2"]
    TN["Translator<br/>Agent N"]

    O --> T1
    O --> T2
    O --> TN
```

**Execution Model**: Translate independent files or sections in parallel when the active runtime supports parallel agents. If parallel dispatch is unavailable, process the manifest sequentially with the same prompts and filesystem outputs.

---

## Model Selection

### Default (no flags)
| Task | Model |
|------|-------|
| Content translation | Standard translation-capable model |
| Metadata/TOC | Fast model |
| Validation | Fast model |

### With `--high-quality`
| Task | Model |
|------|-------|
| Content translation | Strongest practical model |
| Metadata/TOC | Stronger model |
| Validation | Stronger model |

### Automatic Upgrade
- If quality score < 70: Re-translate flagged files with a stronger model
- If translation fails: Retry with upgraded model

---

## Execution Workflow

### Phase 1: Analysis

1. Create work directory:
   ```bash
   WORK_DIR="/tmp/epub_translate_$(date +%s)"
   mkdir -p "$WORK_DIR"/{extracted,sections,translated,status,logs}
   ```

2. Analyze EPUBs (with configurable split threshold):
   ```bash
   python3 scripts/analyze_epub.py \
     --epub "{EPUB_PATH}" \
     --work-dir "$WORK_DIR" \
     --source-lang "{SOURCE_LANG}" \
     --target-lang "{TARGET_LANG}" \
     --split-threshold 30 \
     --split-parts 4
   ```

3. Review `$WORK_DIR/manifest.json` for task count.

### Phase 2: Translation

1. Select translator prompt from [references/](references/):
   - Japanese: `translator_ja.md`
   - English: `translator_en.md`
   - Other: `translator_generic.md`

2. Dispatch translation work in batches:
   - Use parallel agents if the runtime supports them
   - Otherwise translate each manifest entry sequentially
   - Process in batches of `--parallel` count when parallelism is available
   - Require each task to write its translated output file and status file

3. Batch execution pattern:
   ```
   For each batch of N tasks:
     - Dispatch N translation jobs when parallelism is available
     - Confirm each expected output file exists
     - Track completed/failed tasks
     - Proceed to next batch
   ```

4. Retry failed tasks (max 2 attempts, use a stronger model if persistent)

### Phase 3: Finalization

1. Merge split files:
   ```bash
   python3 scripts/merge_xhtml.py --work-dir "$WORK_DIR" --manifest manifest.json
   ```

2. Translate metadata and navigation (LLM-based):
   - Run a metadata translation pass using `translator_metadata.md`
   - Translate: toc.ncx, nav.xhtml, content.opf (title, author, description)
   - Translate: cover.xhtml, titlepage.xhtml (if present)
   - Ensure TOC entries match translated chapter headings

3. **Apply layout conversion** (CRITICAL - must be done before packaging):

   Determine conversion type based on target language and `--vertical` option:

   | Target Language | `--vertical` | Result |
   |-----------------|--------------|--------|
   | ko, en, etc. | (ignored) | horizontal-tb, ltr |
   | ja, zh | false (default) | horizontal-tb, ltr |
   | ja, zh | true | vertical-rl, rtl (우종서/縦書き) |
   | ar, he, fa | (ignored) | horizontal-tb, rtl |

   **A. Horizontal output (default for all languages):**
   ```bash
   TRANSLATED_DIR="$WORK_DIR/translated/{VOLUME_ID}"

   # Convert CSS files: vertical-rl → horizontal-tb
   find "$TRANSLATED_DIR" -name "*.css" -exec sed -i '' \
       -e 's/writing-mode:[[:space:]]*vertical-rl/writing-mode: horizontal-tb/g' \
       -e 's/-webkit-writing-mode:[[:space:]]*vertical-rl/-webkit-writing-mode: horizontal-tb/g' \
       -e 's/-epub-writing-mode:[[:space:]]*vertical-rl/-epub-writing-mode: horizontal-tb/g' \
       {} \;

   # Convert content.opf: page direction and writing mode
   find "$TRANSLATED_DIR" -name "content.opf" -exec sed -i '' \
       -e 's/page-progression-direction="rtl"/page-progression-direction="ltr"/g' \
       -e 's/primary-writing-mode" content="vertical-rl"/primary-writing-mode" content="horizontal-tb"/g' \
       {} \;

   # Convert XHTML inline styles if present
   find "$TRANSLATED_DIR" -name "*.xhtml" -exec sed -i '' \
       -e 's/writing-mode:[[:space:]]*vertical-rl/writing-mode: horizontal-tb/g' \
       {} \;
   ```

   **B. Vertical output (only when `--vertical` AND target is ja/zh):**
   ```bash
   TRANSLATED_DIR="$WORK_DIR/translated/{VOLUME_ID}"

   # Convert CSS files: horizontal-tb → vertical-rl
   find "$TRANSLATED_DIR" -name "*.css" -exec sed -i '' \
       -e 's/writing-mode:[[:space:]]*horizontal-tb/writing-mode: vertical-rl/g' \
       -e 's/-webkit-writing-mode:[[:space:]]*horizontal-tb/-webkit-writing-mode: vertical-rl/g' \
       -e 's/-epub-writing-mode:[[:space:]]*horizontal-tb/-epub-writing-mode: vertical-rl/g' \
       {} \;

   # Convert content.opf: page direction and writing mode for vertical
   find "$TRANSLATED_DIR" -name "content.opf" -exec sed -i '' \
       -e 's/page-progression-direction="ltr"/page-progression-direction="rtl"/g' \
       -e 's/primary-writing-mode" content="horizontal-tb"/primary-writing-mode" content="vertical-rl"/g' \
       {} \;

   # Convert XHTML inline styles if present
   find "$TRANSLATED_DIR" -name "*.xhtml" -exec sed -i '' \
       -e 's/writing-mode:[[:space:]]*horizontal-tb/writing-mode: vertical-rl/g' \
       {} \;
   ```

   **C. RTL output (for ar/he/fa targets):**
   ```bash
   # Convert page direction
   sed -i '' 's/page-progression-direction="ltr"/page-progression-direction="rtl"/g' "$TRANSLATED_DIR"/content.opf

   # Convert CSS direction
   find "$TRANSLATED_DIR" -name "*.css" -exec sed -i '' \
       -e 's/direction:[[:space:]]*ltr/direction: rtl/g' \
       {} \;
   ```

   **Note**: If source is already vertical and `--vertical` is set, skip CSS conversion (keep existing vertical layout).

   See `references/layout_conversion.md` for complete conversion patterns.

4. Verify source text removed:
   ```bash
   python3 scripts/verify.py --work-dir "$WORK_DIR" --source-lang "{SOURCE_LANG}"
   ```

### Phase 4: Quality Validation (LLM-Based)

1. Extract text for validation (token-efficient format):
   ```bash
   python3 scripts/extract_for_validation.py \
     --dir "$WORK_DIR/translated" \
     --output-dir "$WORK_DIR/validation" \
     --max-tokens 8000
   ```

2. Select validator prompt from [references/](references/):
   - Korean target: `validator_ko.md` (extends `validator_generic.md`)
   - Other targets: `validator_generic.md`

3. Spawn validation Task agents in **foreground mode** (batched):
   - Read `$WORK_DIR/validation/validation_manifest.json`
   - For each chunk, spawn a validator agent with:
     - `model: "haiku"` (sufficient for validation)
   - **CRITICAL**: Multiple Tasks in single message for parallel execution
   - Process in batches, collect results directly

4. Aggregate results:
   - Collect validation results from agent responses
   - Calculate average quality score
   - Identify files flagged for re-translation

5. **If average score < 70**: Re-translate flagged files with `model: "opus"`

### Phase 5: Packaging

1. Package EPUB:
   ```bash
   bash scripts/package_epub.sh "$WORK_DIR" "{OUTPUT_DIR}"
   ```

2. Generate final report with quality metrics

---

## File Splitting Configuration

Conservative defaults prevent context overflow in translation agents:

| Setting | Default | Description |
|---------|---------|-------------|
| `split-threshold` | 30 KB | Files larger than this are split |
| `split-parts` | 4 | Number of sections per large file |

### Tuning Guidelines

- **Slow connection / Timeouts**: Lower threshold (20 KB), more parts (6)
- **Fast connection / Large context**: Higher threshold (50 KB), fewer parts (3)
- **Very large files (100KB+)**: Will be split into more parts automatically

---

## Quality Validation (LLM-Based)

Translation quality is validated by an LLM-based review pass, not regex patterns. This provides:
- Context-aware naturalness assessment
- Understanding of literary style and tone
- Detection of subtle translation issues

### Validator Instructions

| Target Language | Primary Instruction | Base Instruction |
|-----------------|---------------------|------------------|
| Korean | `validator_ko.md` | `validator_generic.md` |
| Other | `validator_generic.md` | - |

### Korean-Specific Checks
- **Translationese (번역투)**: `~하는 것이다`, `~라고 하는`, etc.
- **Pronoun overuse**: Excessive `그녀는`, `그는`
- **Particle chains**: Awkward `의의의` patterns
- **Honorific consistency**: Speech level matching

### Quality Score
- 90-100: Excellent - reads naturally
- 75-89: Good - minor issues
- 60-74: Acceptable - review recommended
- <60: Poor - re-translation needed

### Validation Workflow
1. Text extracted in token-efficient format
2. Chunked for parallel validation (8000 tokens each)
3. LLM validators spawned in foreground batches
4. Results collected directly from agent responses
5. Results aggregated into final report

---

## Language-Specific Processing

### Source Language Handling

| Source | Special Handling |
|--------|------------------|
| Japanese | Remove ruby tags, handle vertical writing |
| Chinese | Handle traditional/simplified, remove pinyin |
| Arabic/Hebrew | Handle RTL text direction |
| English | Standard processing |

### Layout Conversion (Target-Based)

**Key Principle**: All languages default to horizontal LTR (except RTL languages).

| Target Language | Page Direction | Writing Mode | Text Direction | Notes |
|-----------------|----------------|--------------|----------------|-------|
| Korean (ko) | ltr | horizontal-tb | ltr | |
| English (en) | ltr | horizontal-tb | ltr | |
| Japanese (ja) | ltr | horizontal-tb | ltr | Default |
| Japanese (ja) + `--vertical` | rtl | vertical-rl | ltr | 縦書き (우종서) |
| Chinese (zh) | ltr | horizontal-tb | ltr | Default |
| Chinese (zh) + `--vertical` | rtl | vertical-rl | ltr | 縱排 (우종서) |
| Arabic (ar) | rtl | horizontal-tb | rtl | |
| Hebrew (he) | rtl | horizontal-tb | rtl | |

**Note**: `--vertical` option is only valid for Japanese (ja) and Chinese (zh) targets. It will be ignored for other languages.

See `references/layout_conversion.md` for complete conversion scripts.

---

## Custom Dictionary (Optional)

**The translator works without external dictionary files.** It naturally translates based on context.

Use custom dictionaries ONLY for:
- **Proper nouns**: names, places, organizations, brands
- **Document-specific terms**: proprietary terms unique to this document

**Do NOT add common words** - let the translator handle them naturally.

### Creating a Custom Dictionary

See [assets/template.json](assets/template.json) for format:

```json
{
  "proper_nouns": { "names": { "田中太郎": "Tanaka Taro" } },
  "domain_terms": { "ProprietaryTech": "고유 기술명" }
}
```

### Academic/Technical Template

For academic or technical documents, use [assets/template_academic.json](assets/template_academic.json).

---

## Work Directory Structure

```
$WORK_DIR/
├── manifest.json           # Task manifest
├── extracted/              # Extracted EPUB contents
├── sections/               # Split large files
├── translated/             # Translated files
├── validation/             # Validation input/output files
│   ├── validation_manifest.json
│   ├── validate_001_input.txt
│   ├── validate_001_result.json
│   └── ...
├── status/                 # Task status files
└── logs/                   # Log files
```

## Status Codes

| Status | Meaning |
|--------|---------|
| `pending` | Not started |
| `in_progress` | Being translated |
| `completed` | Done |
| `failed` | Error occurred |

---

## Error Handling

| Error | Action |
|-------|--------|
| Extraction failure | Skip corrupted file |
| Translation timeout | Split further, retry |
| XML error | Attempt fix, report |
| Remaining source text | Re-translate or manual review |
| Low quality score | Review samples, re-translate if needed |

---

## File Reference

| Path | Description |
|------|-------------|
| `SKILL.md` | This file |
| `references/orchestrator.md` | Detailed orchestrator instructions |
| `references/translator_*.md` | Language-specific translator prompts |
| `references/translator_metadata.md` | Metadata and TOC translation instruction |
| `references/layout_conversion.md` | Writing direction and layout conversion guide |
| `references/validator_generic.md` | Generic validation instruction |
| `references/validator_ko.md` | Korean-specific validation instruction |
| `scripts/analyze_epub.py` | EPUB analysis (configurable splitting) |
| `scripts/split_xhtml.py` | File splitting |
| `scripts/merge_xhtml.py` | Section merging |
| `scripts/verify.py` | Source text verification |
| `scripts/extract_for_validation.py` | Token-efficient text extraction for LLM validation |
| `scripts/package_epub.sh` | EPUB packaging |
| `assets/template.json` | Dictionary template |
| `assets/template_academic.json` | Academic dictionary template |