I recall that Nielsen Norman Group published 6 Tips for Improving Language Switchers on Ecommerce Sites based on a study they did.
The give the following tips for language switchers in general:
- Default to the language version that matches users’ browser settings.
- On desktop sites, put the language switcher in the top corners (left or right).
- On mobile sites, put the language switcher above the fold or inside the navigation menu.
- Display the name of an available language in that language (e.g., Español instead of Spanish).
- Use a combination of symbols to represent the switch (but not country flags)
Unfortunately it does not cover the issue of fractional translations.
How we improved translations on GOV.UK explains that from a more technical perspective, in that exact context where only certain articles were translated, but not the surrounding menus etc. but there is no user research background. Their design system team unfortunately didn’t write on the subject.
To NN/g first tip I’d like to add that language should be dependent on the URL, so that search engines can direct users to the article in the language they picked. One good pattern is to add a banner if the browser language does not match the content language, offering the content in that language. After all, the user might understand that language perfectly fine.