+                      (entry (tag "tag-for-first-news-entry")
+                             (title (en "Old news.") (eo "Malnova?oj."))

The question mark here suggests you’re not running the tests with a
UTF-8 locale.

Could you add, say, ‘glibc-locales’ to your environment, ensure
GUIX_LOCPATH points to it, and set LC_ALL=en_US.UTF-8 (or similar)?

Two comments:

If tests require an UTF-8 locale, I think the tests (maybe in build-aux/test-driver.scm?) should check that an UTF-8 that an UTF-8 locale is actually in use and otherwise bail out properly.

It's not a file-name but rather the contents of the news file, so I would think we are just forgetting to pass some arguments like #:encoding "UTF-8" -- making the interpretation of the news file depend on the current locale doesn't seem good to me (it's encoding on stdout with "guix pull --news" is another matter).

Greetings,
Maxime.