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

[Cubel89]

What this PR adds

Internationalization (i18n) support for the periodic table data through separate translation files, without modifying any existing files.

Structure

A new i18n/ directory with:

• README.md — Documentation and contribution guide
• en-US.json — English (United States)
• en-GB.json — English (United Kingdom)
• es-ES.json — Spanish (Spain)
• es-AR.json — Spanish (Argentina)
• pt-PT.json — Portuguese (Portugal)

Each translation file includes:

• Element names (all 119 elements)
• Categories (all 15 categories)
• Phases (Gas, Liquid, Solid)
• Summaries (all 119 element descriptions)
• Appearances (all 59 unique appearance values)

All translations are indexed by atomic number and use the original English values as keys for categories, phases, and appearances.

Why separate files?

• Zero changes to existing files — no risk of breaking current consumers
• Easy to contribute: one file per language, no merge conflicts between translators
• Scales well: adding a new language is just adding a new file
• Follows standard i18n patterns used across the industry

Language-specific highlights

• en-US / en-GB: Consistent spelling for each variant (Aluminum vs Aluminium, color vs colour, gray vs grey)
• es-ES: Official IUPAC Spanish nomenclature (RAE)
• es-AR: Rioplatense Spanish voice, independently translated from English (not adapted from es-ES)
• pt-PT: European Portuguese conventions (e.g., "Azoto" for Nitrogen, not Brazilian "Nitrogénio")

How to use

const elements = require('./PeriodicTableJSON.json');
const esES = require('./i18n/es-ES.json');                                                                                                                                     
                                                                                                                                                                               
const hydrogen = elements.elements[0];                                                                                                                                         
const name_es = esES.names[String(hydrogen.number)]; // "Hidrógeno"                                                                                                            
const category_es = esES.categories[hydrogen.category]; // "no metal diatómico"                                                                                                
                                                                                                                                                                               
Quality                                                                                                                                                                        
                                                                                                                                                                               
- Official IUPAC nomenclature used for each language                                                                                                                           
- Native-quality writing, not machine translation
- All diacritics and special characters correct                                                                                                                                
- Each regional variant respects its own conventions                                                                                                                           
                                                                                                                                                                               
Contributing new languages                                                                                                                                                     
                                                                                                                                                                               
The i18n/README.md includes a complete guide for contributors who want to add new languages.