Internationalization + Finnish language

[iqqmuT]

• refers to Translations of the website #75
• uses next-intl
• locale auto-detected from user's browser by default
• supported locales (in this PR):
  • en-US (fallback)
  • fi
• locale used for number formatting
• locale used for currency symbol positioning
• changed locale is stored into a cookie
• URL paths are not changed so old shared URLs keep working
• easy to add a new locale:
  • create new file messages/LOCALE.json
  • add locale to src/i18n.ts

[scastiel]

@iqqmuT This is so amazing! Let me review and test everything and let's get it merged! Internationalization is the # 1 asked feature for Spliit 🤩