You can now preview any user interface/content language by adding
?uselang=<language code>
to the end of the URL. This is useful for sending users links in a given language, and for testing translations. It only applies to the page you’re loading (but can be used to compose a review in the given language).
Example:
https://lib.reviews/?uselang=es - frontpage in Spanish
The list of language codes can be found here; some aren’t activated yet since they’re incomplete.