test: ensure FORCED_LANGUAGE works

Closes #1077

test/forced-language/test.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+function cleanup() {
+  pkill -P $$
+}
+
+trap cleanup EXIT SIGINT
+
+# Build static assets
+(cd ../.. && npm ci && npm run assets)
+
+go tool anubis --help 2>/dev/null ||:
+
+go run ../cmd/unixhttpd &
+
+FORCED_LANGUAGE=de go tool anubis \
+  --policy-fname ./anubis.yaml \
+  --use-remote-address \
+  --target=unix://$(pwd)/unixhttpd.sock &
+
+backoff-retry node ./test.mjs