Fix i18n and layouts

• Check the condition (> 50 translations) and update languages at each build, generate menus and categories at build time;
• Use suitable language codes so that the language list is not messed up anymore;
• Category list and single layouts now work again
• Use site title and site description (in config.yaml)
• Cleanup old files

Main changes are in 5dbed2c4

This MR needs to go with sysadmin/ci-tooling!193 (merged).