Add translations for ar-AR, de-DE, fr-FR, it-IT, pt-BR, ru-RU and ena… (#4572)

…ble in frontend

- Updated ar-AR (Arabic) to 98.7% completion (1088 entries)
- Updated fr-FR (French) to 97.3% completion (1296 entries)
- Updated pt-BR (Portuguese Brazil) to 98.6% completion (1294 entries)
- Updated ru-RU (Russian) to 98.1% completion (1277 entries)
- Updated ja-JP (Japanese) to 73.4% completion (796 entries, batches
1-2)
- Updated es-ES minor corrections
- Enabled 8 languages with >90% completion in LanguageSelector
- Added JSON validation scripts for translation quality assurance
- RTL support already enabled for ar-AR

Enabled languages: en-GB, ar-AR, de-DE, es-ES, fr-FR, it-IT, pt-BR,
ru-RU, zh-CN

🤖 Generated with [Claude Code](https://claude.com/claude-code)

# 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)
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---

## 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.

Co-authored-by: Claude <[email protected]>

frontend/public/locales/es-ES/translation.json

@@ -38,7 +38,7 @@
   "save": "Guardar",
   "saveToBrowser": "Guardar en el navegador",
   "close": "Cerrar",
-  "filesSelected": "archivos seleccionados",
+  "filesSelected": "{{count}} archivos seleccionados",
   "noFavourites": "No se agregaron favoritos",
   "downloadComplete": "Descarga completada",
   "bored": "¿Aburrido de esperar?",

frontend/src/components/shared/LanguageSelector.tsx

@@ -269,8 +269,9 @@ const LanguageSelector: React.FC<LanguageSelectorProps> = ({ position = 'bottom-
           <ScrollArea h={190} type="scroll">
             <div className={styles.languageGrid}>
               {languageOptions.map((option, index) => {
-                const isEnglishGB = option.value === 'en-GB'; // Currently only English GB has enough translations to use
-                const isDisabled = !isEnglishGB;
+                // Enable languages with >90% translation completion
+                const enabledLanguages = ['en-GB', 'ar-AR', 'de-DE', 'es-ES', 'fr-FR', 'it-IT', 'pt-BR', 'ru-RU', 'zh-CN'];
+                const isDisabled = !enabledLanguages.includes(option.value);
 
                 return (
                   <LanguageItem