feat: add i18n translations (es-ES, pt-PT)

i18n/README.md

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+# Translations (i18n)
+
+Translations of element names, categories, and phases into multiple languages.
+
+## File format
+
+Each language has its own JSON file following the naming convention `{language}-{REGION}.json` (e.g., `es-ES.json`) or `{language}.json` for languages without regional variants (e.g., `ca.json`).
+
+Language codes follow [ISO 639-1](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) and region codes follow [ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2).
+
+### Structure
+
+```json
+{
+  "meta": {
+    "language": "es-ES",
+    "language_name": "Spanish (Spain)",
+    "native_name": "Castellano",
+    "translator": "GitHub username or name",
+    "last_updated": "2026-03-31",
+    "source": "References used for the translation",
+    "completeness": {
+      "names": true,
+      "categories": true,
+      "phases": true,
+      "summaries": false
+    }
+  },
+  "names": {
+    "1": "Hidrógeno",
+    "2": "Helio",
+    "3": "Litio"
+  },
+  "categories": {
+    "diatomic nonmetal": "no metal diatómico",
+    "noble gas": "gas noble"
+  },
+  "phases": {
+    "Gas": "Gas",
+    "Liquid": "Líquido",
+    "Solid": "Sólido"
+  }
+}
+```
+
+### Fields
+
+| Field | Required | Description |
+|-------|----------|-------------|
+| `meta` | Yes | Metadata about the translation |
+| `meta.language` | Yes | Language code (ISO 639-1 + optional ISO 3166-1 region) |
+| `meta.language_name` | Yes | Language name in English |
+| `meta.native_name` | Yes | Language name in its own language |
+| `meta.translator` | Yes | Who made the translation |
+| `meta.last_updated` | Yes | Date of last update (YYYY-MM-DD) |
+| `meta.source` | Yes | References used (e.g., "IUPAC + RAE") |
+| `meta.completeness` | Yes | Which sections are complete |
+| `names` | Yes | Element names indexed by atomic number (1-119) |
+| `categories` | Yes | Translation of the 15 element categories |
+| `phases` | Yes | Translation of the 3 phases (Gas, Liquid, Solid) |
+| `summaries` | No | Element summaries indexed by atomic number (future) |
+
+### Key conventions
+
+- **Names** are indexed by atomic number as strings (`"1"`, `"2"`, ..., `"119"`)
+- **Categories** use the original English category as the key
+- **Phases** use the original English phase as the key
+
+## Available translations
+
+| Language | Code | File | Translator |
+|----------|------|------|------------|
+| Spanish (Spain) | `es-ES` | [es-ES.json](es-ES.json) | Cubel89 |
+
+## Contributing a new language
+
+1. Copy an existing translation file (e.g., `es-ES.json`) and rename it with the appropriate language code
+2. Update the `meta` section with your language details
+3. Translate all 119 element names, 15 categories, and 3 phases
+4. Use official nomenclature from your country's chemical society or IUPAC when available
+5. Open a pull request with your translation
+
+### Quality guidelines
+
+- Use official chemical nomenclature from recognized institutions (IUPAC, national chemistry societies)
+- Ensure proper diacritics and special characters for your language
+- Do not use machine translation without manual review by a native speaker
+- Verify elements 113-119 (named in 2016) have their official translated names
+
+### Recommended sources
+
+- [IUPAC Nomenclature](https://iupac.org/what-we-do/nomenclature/)
+- [Wikipedia](https://wikipedia.org) in the target language
+- [Ptable.com](https://ptable.com) (supports 40+ languages)
+- [Elementymology](https://elements.vanderkrogt.net/multidict.php) (80+ languages)
+- National chemistry societies and language academies