Add default language selecting with localStorage

Now, Slate will use localStorage to track the last selected
language, so if the user visits again with no language in the URL,
they will be redirected to the last language they used.

This update also fixes a bug with how the layout inserts the
language names into the javascript. Previously, they would be,
(for some inane reason) arrays within arrays. It is now just
an array. Whoever wrote that previous code was clearly a fool.