[i18n_subsites] Possibility to hide separately untranslated pages and articles

It may be useful to hide only untranslated articles, or only untranslated pages.
For instance one can consider the page lang consistency to be more important on a site, while allowing some articles to be in another language.

I used to hide pages manually with the theme, but maybe this way is more appropriate.

Sorry for the duplicate PR, I am new to github…