lang updates plus --include-existing flag (#5212)

# Description of Changes

<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
-->

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

scripts/translations/README.md

@@ -174,9 +174,6 @@ Merges missing translations from en-GB into target language files and manages tr
 # Add missing translations from en-GB to French
 python scripts/translations/translation_merger.py fr-FR add-missing
 
-# Add without marking as [UNTRANSLATED]
-python scripts/translations/translation_merger.py fr-FR add-missing --no-mark-untranslated
-
 # Extract untranslated entries to a file
 python scripts/translations/translation_merger.py fr-FR extract-untranslated --output fr_untranslated.json
 
@@ -188,7 +185,7 @@ python scripts/translations/translation_merger.py fr-FR apply-translations --tra
 ```
 
 **Features:**
-- Adds missing keys from en-GB with optional [UNTRANSLATED] markers
+- Adds missing keys from en-GB (copies English text directly)
 - Extracts untranslated entries for external translation
 - Creates structured templates for AI translation
 - Applies translated content back to language files
@@ -442,7 +439,7 @@ Repeat steps 2-5 until 100% complete.
 
 #### Step 1: Add Missing Translations
 ```bash
-python scripts/translations/translation_merger.py fr-FR add-missing --mark-untranslated
+python scripts/translations/translation_merger.py fr-FR add-missing
 ```
 
 #### Step 2: Create AI Template
@@ -523,7 +520,7 @@ ignore = [
 
 ### Critical Rules for Translation
 
-1. **NEVER skip entries**: Translate ALL entries in each batch to avoid [UNTRANSLATED] pollution
+1. **NEVER skip entries**: Translate ALL entries in each batch to ensure completeness
 2. **Use appropriate batch sizes**: 100 entries for systematic translation, unlimited for compact method
 3. **Skip validation for placeholders**: Use `--skip-validation` when batch contains `{{variable}}` patterns
 4. **Check progress between batches**: Use `--summary` flag to track completion percentage
@@ -567,13 +564,6 @@ python scripts/translations/json_validator.py --all-batches ar_AR
 - Regex patterns: Double all backslashes (`\d` → `\\d`)
 - Check for missing/extra commas at line reported in error
 
-#### [UNTRANSLATED] Pollution
-**Problem**: Hundreds of [UNTRANSLATED] markers from incomplete translation attempts
-**Solution**:
-- Only translate complete batches of manageable size
-- Use analyzer that counts [UNTRANSLATED] as missing translations
-- Restore from backup if pollution occurs
-
 #### Validation False Positives
 **Problem**: Validator flags legitimate `{{variable}}` placeholders as artifacts
 **Solution**: Use `--skip-validation` flag when applying batches with template variables
@@ -674,7 +664,7 @@ python scripts/translations/ai_translation_helper.py apply-batch de_batch_1.json
 - **Missing Files**: Scripts create new files when language directories don't exist
 - **Invalid JSON**: Clear error messages with line numbers
 - **Placeholder Mismatches**: Validation warnings for missing or extra placeholders
-- **[UNTRANSLATED] Entries**: Counted as missing translations to prevent pollution
+- **Legacy [UNTRANSLATED] Markers**: Detected and stripped for backwards compatibility
 - **Backup Failures**: Graceful handling with user notification
 
 ## Integration with Development