Support for "multi-linguality" across webpages

[mukil]

The main idea to implement mulit-linguality is now like this:

• We utilize the multi-site capabilities of the dmx-webpages plugin, one Website for each language available
• We associate Webpages in between the different Websites using "Translation" associations

Extensions to the Application Model:

• Introduce "Website Language" type (Text) with a set of pre-installed BCP47 language tags (https://tools.ietf.org/rfc/bcp/bcp47.txt)
• Add "Website Language" to "Website" type definitions
• Introduce new assoc type "Translation" (Entity), with "Website Language" as a child topic

Add server-side logic:

• Each webpage requested, with "Translation" associations, gets displayed with a dropdown menu / simple menu in the footer rendering a human readable "Label" for each existing "Website Language" tag associated with a "Webpage". Clicking that translation label, will request the visitor the associated "Webpage" (and therefore may switch Websites implicitly)
• Each top level html element of a webpage inherits the "Website Language" value set from its associated Website topic (e.g. html lang="en" is added as an attribute.

Maybe, at a later stage, support for "Content negotiation" will be added server-side:

• Each request to a webpage/website is parsed for a language preference through inspecting the values in the Accept-Language HTTP header sent by the visitors browser.