Add project documentation and translation guide (#174)

docs/translation-guide.md

@@ -0,0 +1,77 @@
+# Language Guide
+
+## Adding/Updating Translations
+
+### Adding a New Language
+
+To add a new language translation to HabitTrove:
+
+1. **Create translation file**: 
+   - Copy `messages/en.json` as a template
+   - Save as `messages/{language-code}.json` (e.g., `ko.json` for Korean)
+   - Translate all values while preserving keys and placeholder variables like `{username}`, `{count}`, etc.
+
+2. **Validate translation structure**:
+   ```bash
+   # Ensure JSON is valid
+   jq empty messages/{language-code}.json
+   
+   # Compare structure with English (should show no differences)
+   diff <(jq -S . messages/en.json | jq -r 'keys | sort | .[]') <(jq -S . messages/{language-code}.json | jq -r 'keys | sort | .[]')
+   ```
+
+3. **Add language option to UI**:
+   - Edit `app/settings/page.tsx`
+   - Add new `<option value="{language-code}">{Language Name}</option>` in alphabetical order
+
+4. **Update documentation**:
+   - Add language to README.md supported languages list
+   - Create new changelog entry with version bump
+   - Update package.json version
+
+### Example: Adding Korean (한국어)
+
+```bash
+# 1. Copy translation file
+cp /path/to/ko.json messages/ko.json
+
+# 2. Add to settings page
+# Add: <option value="ko">한국어</option>
+
+# 3. Update README.md
+# Change: 简体中文, 日본語
+# To: 简체中文, 한국어, 日본語
+
+# 4. Add changelog entry
+# Create new version section with language addition
+
+# 5. Bump package version
+# Update version in package.json
+```
+
+### Translation Quality Guidelines
+
+- Use natural, contextually appropriate expressions
+- Maintain consistent terminology throughout
+- Preserve all placeholder variables exactly: `{username}`, `{count}`, `{target}`, etc.
+- Use appropriate formality level for the target language
+- Ensure JSON structure matches English file exactly (385 total keys)
+
+### Validation Commands
+
+```bash
+# Check JSON validity
+jq empty messages/{lang}.json
+
+# Compare key structure
+node -e "
+const en = require('./messages/en.json');
+const target = require('./messages/{lang}.json');
+// ... deep key comparison script
+"
+
+# Verify placeholder consistency
+grep -o '{[^}]*}' messages/en.json | sort | uniq > en_vars.txt
+grep -o '{[^}]*}' messages/{lang}.json | sort | uniq > {lang}_vars.txt
+diff en_vars.txt {lang}_vars.txt
+```