Multi-language enhancement

[NicoPy]

To enhance multi-language support :

• Add global translation buttons as described in i18n_subsites plugin documentation.
• Add a PAGE_HIDE_TRANSLATION option like the ARTICLE_HIDE_TRANSLATION and use it in page.html template.
• Modify CSS accordingly.

This allows to switch language from any page, even those that are neither articles nor pages.

[alexandrevicenzi]

Feel free to send a PR. i18n was never a major thing in this theme, only i10n.

[JulienPalard]

I'm not fan of those global translation buttons as documented by i18n_subsites: they always lead to the homepage, which is surprising, as a user I expect them to give me the same page but translated.

I would not be happy with "global translation buttons" leading to the same URL on the translated subsite, as not every pages and articles may exist in the translations, those button may lead to unexisting pages.

What I think would work great is to add translation buttons on just a few more templates, and for consistency display them like on articles and pages.

Typically the index template could benefit from those buttons as it's translated, maybe some other like archive, authors, categories and tags may be interesting too.

Something simple like this should do the work:

{% if lang_siteurls %}
  <div class="translations">
    {% for lang, url in lang_siteurls.items() %}
      <a href="{{ url }}/">{{ lang }}</a>
    {% endfor %}
  </div>
{% endif %}

But it requires touching the style.less at least to make the button appear on the right (for consistency with the article style).