Basic support for multi language sites

Basic support for multi language sites. Users configure "defaultLanguage" and "availableLanguages" in their settings.json. If a folder has both project.md and project.langCode.md, the additional contentFiles are parsed and added as sub pages to the mother page (page.translatedCopies). These translated copies link to the correct translated parents and children. If a translation is missing, it falls back on to the default language.

To access a translated version of the site 'langcode/' is prefixed to the uri (eg. www.example.com/langCode/project). Slug's are not translated. 

One could add more features, such as automatic language negotiation (I personally find this annoying).