Kolibri public APIs aren't properly ignoring language settings

[bjester]

Observed behavior

The Kolibri public API is respecting the UI language cookie (django_language) causing error messages, like that for a 404, to be returned in non-English.

Expected behavior

The APIs are cached through Cloudflare, so the response shouldn't change based off any user request info.

Steps to reproduce the issue

1. Change the UI language to something other than english
2. Query the public API for something that doesn't exist, e.g. https://unstable.studio.learningequality.org/api/public/v2/contentnode/8e6574944e1949c197faa22ff3c918dc/
3. Observe the detail message isn't in English

[rtibbles]

Fixed in #4198