Simplify multi-language support

Created by: MaxXor

The current multi-language support is hard to maintain and gets out of sync very fast. A good example how easy it can be implemented in Jekyll can be seen in https://github.com/bitcoinxt/website/pull/51. I propose that we adapt this to make it less error-prone to update or add pages to the website.

They use jekyll-multiple-languages-plugin and have one layout for all languages which contain placeholders like {% t page.title %}. Beside that the language files are simple .yml-files which contain the translated strings.

This issue can be used to track progress on simplifying multi-language support.