diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 242b803b..9bab7df3 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -158,6 +158,12 @@ jobs:
     env:
       # MUST match GRADLE_USER_HOME in .github/ci-image/jvm-test.Dockerfile.
       GRADLE_USER_HOME: /opt/gradle-home
+      # The image drops /usr/share/locale, so the JVM inherits an ASCII `sun.jnu.encoding` and cannot create
+      # a file whose name is not ASCII — `DiffPresenterIsWithinRootTest` writes `fïle ñ.txt` and dies with
+      # FileNotFoundException. This gate runs the SAME suite as ci.yml, so it needs the SAME locale: a job
+      # that runs `test` without this is green on the branch and red on the release. `C.UTF-8` is built into
+      # glibc, so it needs no image rebuild.
+      LANG: C.UTF-8
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: