Set language according to HTTP_REFERER for links from OCDS documentation

[ghost]

• Starting at https://standard.open-contracting.org/infrastructure/latest/es/ and clicking the review link takes you to the English version.
• Same for https://standard.open-contracting.org/latest/es/

(Found when doing open-contracting/deploy#53)

[ghost]

Suggestions:

There is a global "language" variable in Sphinx. https://www.sphinx-doc.org/en/master/templating.html#global-variables Oddly, it comes out as None for English, but the others are fine - "es", "fr" and "it"

We can use this to edit the template in the standard and attach a language code to the link to the review tool. We can use a if statement to look for English and put in en. https://github.com/open-contracting/standard/blob/1.1-dev/docs/_templates/layout.html#L25 and https://github.com/open-contracting/infrastructure/blob/0.9-dev/docs/_templates/layout.html#L15

Then it's just a case of making something in Cove/Django that responds to that:

• The Django way would be to add i18n_patterns to our urls: https://docs.djangoproject.com/en/3.1/topics/i18n/translation/#language-prefix-in-url-patterns
• If we wanted to avoid many URL's changing, it could just be a get parameter and we write some Python code in the view to respond to it manually.

Note: The main standard site has more languages then Cove has (ie; fr, it) - have to test that Django will use a suitable fallback in this case.

However, this involves changing the standard site which is of course versioned.

So another route may be to ignore all this:

Links should have a HTTP_REFERER set. In Django index view, look for a referrer, take it apart to get the languge, set the Django language in Python. No changes to Standard required now, no changes to standard required if we add or remove a language to a Cove later.

[jpmckinney]

The link can also be changed with JavaScript to send a POST request to the language redirect view, setting language to 'en' or 'es' and next to the /review/ URL.

Alternately, we can directly set the cookie, but that seems more tightly coupled, since it's not expected to change another app's cookies (whereas it's expected to request another app's URLs).

[jpmckinney]

Noting that since the URL is in the translated string (and not passed to e.g. blocktranslate as a variable), it's easy to hand-code different links for different languages via gettext/Transifex. This avoids needing to have consistent language codes across Sphinx and CoVE, and to have logic about which languages are available in CoVE.

The review tools are now on sub-domains of standard.open-contracting.org. It's still possible to set shared cookies - with the same caveats as above.

I prefer the HTTP_REFERER solution (essentially treating the OCDS documentation as a special case), in which case this issue can be transferred to cove-ocds.