changelog: Document changes since v27.225

I would have also highlighted English (U.K.) in the user highlights,
except that none of the translation updates for that locale will
actually be visible to the user yet, since we haven't yet turned on
our logic for the "stream" to "channel" rename; see 50c9656.

docs/changelog.md

@@ -39,6 +39,22 @@ It doesn't include
 
 ## Unreleased
 
+### Highlights for users
+
+* Updated translations for Bulgarian, German, Polish, Russian, and
+  Ukrainian. Thanks as always to our kind volunteer translators!
+
+Plus other fixes and improvements for your Zulip experience.
+
+
+### Highlights for developers
+
+* Ran `yarn upgrade`. (PR #5850)
+
+* Disabled Flipper on iOS. (PR #5848)
+
+* Resolved issues: PR #5843
+
 
 ## 27.225 (2024-02-27)