Demonstrate multilanguage capabilities

[JanLenoch]

Motivation

As the source plugin now supports getting language variants of content items, the sample app should also be able to demonstrate this capability.

Design guidelines

• Create a language switch in the index.js page.
• Make the listing of blogpost references, project references and speaking engagements reflect the preferred language.
• Render the detail pages of project references and speaking engagements accordingly.
• Optional: create a language switch in these detail pages as well.

Reference

• https://www.gatsbyjs.org/docs/creating-and-modifying-pages/
• https://developer.kenticocloud.com/docs/localization