Localization with Crowdin

[alexjustesen]

📃 Description

This PR implements localization (translations) using https://crowdin.com.

✅ To-dos

• Update the English (en) file with all known strings.
• Update all strings to use PHP's built-in string localization __('This is a test'), see the Laravel docs for more info.
• Import string file to Crowdin.
• Export existing languages found in /lang from Crowdin.

📖 Resources

• Use Bookstack's structure for language files: https://github.com/BookStackApp/BookStack/tree/development/lang/en

🪵 Changelog

➕ Added

• one
• two

✏️ Changed

• one
• two

🔧 Fixed

• one
• two

🗑️ Removed

• one
• two

📷 Screenshots

If this PR has any UI/UX changes it's strongly suggested you add screenshots here.