provide template for i18n

[Zauberfisch]

This template should also include folders/files and instructions for i18n

Both PHP (/lang/) and JS (/client/lang/)
Though I am not sure about JS, core modules seem to be using some build process, for most modules it might be more practical to just write the .js files manually

• add /lang/readme.md (with example lang file and example usage for php and templates)
• add /client/lang/readme.md (with example lang file, example usage and how to include it)
• expose client/lang in composer.json