One important point for Wikidata is making the information accessible for everybody on all the languages, but for anonymous users I can't see a way to change the language of the user interface. In the language selector if I try to choose another language when I am logged out I see the following message:

Display language: English (same as content)
Log in to select a different language for menus.

I understand that this is probably to prevent having lot of versions of the same page for different languages in the cache because it is preferred to give the same cached version for all the anonymous users, but I think that in this case this situation can be a barrier for the users who don't understand English and don't want to make an account. Also, there already should be more than one version of the page due to the differences on the label and description sections where different languages are displayed to different people, so maybe the impact is smaller than I initially thought.

I think that the best solution (from the perspective of an anonymous user) is using the language preferences of the accept-language header to choose the default language allowing to the user to change it using the language selector to change the language to another one if they prefer.

Similar feature requests:

• T58464: Allow anonymous users to change interface language on Wikimedia wikis with ULS
• T161517: Allow anonymous users to change interface language on Commons with ULS
• T149419: Interface language selection for unregistered users on Wikimedia projects
• T134592: Allow setting the UI to a language other than English for anonymous users

Discussion of the technical background, Varnish caching: T233609: [SPIKE 4hrs] What is technically feasible in terms of logged-in/logged-out users?