Add JavaScript solution to translate 404 pages

[s-gbz]

As of right now there seems to be no solution to output 404 pages in the selected language, since "Jekyll is a static site generator and you can't localize a 404 page without web server's help." ~kurtsson/jekyll-multiple-languages-plugin#156

The solution might be checking the URLs language parameter, "en" or "de" at this point, and change the text or insert a localized 404 image.

The 404 page can be found in the root directory 404.html.