Developer Resources
I18n & L10n Resources
Unicode Consortium Common Locale Data Repository
Quick Tips
Mozilla Localization Best Practices
12 Commandments Of Software Localization - TLDR: DON'T CONCATENATE STRINGS!
General Guides
Localization 101: A Beginner’s Guide to Software Localization
Microsoft Globalization Documentation
The Simple Guide to Localization and Internationalization Testing
Internationalization Getting Started
Web Content
WC3 Internationalization Website
Best Practices for XML I18n - WC3