fix: enforce security profile language across DAVE

lib/llm/dave-persona.mjs

@@ -11,3 +11,19 @@ ADAPTIVE WRITING STYLE:
 - Never imitate spelling mistakes, confusing grammar, hostility, discriminatory language, panic, manipulation, or unjustified certainty.
 - Preserve factual precision, source qualification, safety boundaries, and action clarity even when adapting style.
 - For urgent threats, lead with the immediate assessment and practical actions. Style matching is secondary to comprehension and safety.`;
+
+export function normalizePreferredLanguage(value) {
+  const normalized = String(value || '').trim().toLowerCase();
+  return normalized === 'de' || normalized === 'en' ? normalized : null;
+}
+
+export function preferredLanguagePrompt(value) {
+  const language = normalizePreferredLanguage(value);
+  if (language === 'de') {
+    return 'REQUIRED RESPONSE LANGUAGE: German (de). All user-visible prose in the final answer must be German. Do not switch to English because sources, tools, field names, or prior assistant messages are English.';
+  }
+  if (language === 'en') {
+    return 'REQUIRED RESPONSE LANGUAGE: English (en). All user-visible prose in the final answer must be English unless the operator explicitly requests another language in the current message.';
+  }
+  return 'RESPONSE LANGUAGE: Follow the language of the operator\'s current message.';
+}