Displaying localized information from _data/ entries

[awvwgk]

One open issue for the localization are the entries in _data, some of those are automatically generated, like the fpm registry and therefore hard to translate. Other entries like the in press part of the learning resources might be a suitable candidate for localized information, i.e. displaying French books only on the French subtree.

Note that the default language, English, has only access to the English translation file, while all other languages have at least access to the English translation file and their own translation file, in case we want to pull off some tricks with liquid for this.

Localizing _data entries in the translation file would lead to a significant bloat, but we might be able to add additional localized _data directories in the language subtrees in the _i18n directory. A possible solution would be to use subtrees in the _data dir to store localized versions of data and global versions on the toplevel.

_data
|-- en/
|-- es/
|-- fr/

[awvwgk]

This is resolved now with the new webpage (see #415)