Add localization support (i18n)

[momenbasel]

PureMac currently only supports English. Add localization infrastructure so the community can contribute translations.

Tasks:

• Set up Localizable.strings / String Catalogs
• Extract all user-facing strings
• Add language selection in Settings
• Create documentation for how to contribute translations

Priority languages: Arabic, Spanish, German, French, Japanese, Chinese

Contributions for any language are welcome.