diff --git a/internal/httpapi/phase3.go b/internal/httpapi/phase3.go
index 3f77a54..c2f9a18 100644
--- a/internal/httpapi/phase3.go
+++ b/internal/httpapi/phase3.go
@@ -465,6 +465,9 @@ func executeApprovedAction(ctx context.Context, pool *db.Pool, execID uuid.UUID,
 func provisionScript(pkgs []string, postInstall string) string {
 	var b strings.Builder
 	b.WriteString("set -o pipefail\n")
+	// A fresh debian LXC has no locale set, which spams "Can't set locale"
+	// warnings and breaks some package post-install scripts. Pin C.UTF-8.
+	b.WriteString("export LANG=C.UTF-8 LC_ALL=C.UTF-8 DEBIAN_FRONTEND=noninteractive\n")
 	b.WriteString("probe=deb.debian.org\n")
 	b.WriteString("ok=0\n")
 	b.WriteString("for i in $(seq 1 30); do if getent hosts \"$probe\" >/dev/null 2>&1; then ok=1; break; fi; sleep 2; done\n")