docs: add multilingual output how-to with prompt-based example

Author: apexx1

docs/howto/multilingual.md

@@ -0,0 +1,65 @@
+
+# Multilingual output (how-to)
+
+**Version:** `gdm-concordia >= 2.1.0`
+
+Concordia agents don’t have a global “language” toggle. The recommended way to constrain output language is to **modify the agent’s prompt**. You can do this either by:
+- injecting a language directive into the agent’s params (e.g., `goal`), or
+- wrapping/templating the prompt used by your prefab.
+
+Below is a minimal example that sets one agent to Spanish and another to French.
+
+```python
+from concordia.utils import prefab_lib
+
+LANGUAGE_DIRECTIVE = "Always respond in {lang}. Do not switch languages."
+
+instances = [
+    prefab_lib.InstanceConfig(
+        prefab="basic__Entity",
+        role=prefab_lib.Role.ENTITY,
+        params={
+            "name": "Oliver Cromwell",
+            "goal": "become lord protector. " + LANGUAGE_DIRECTIVE.format(lang="Spanish"),
+        },
+    ),
+    prefab_lib.InstanceConfig(
+        prefab="basic__Entity",
+        role=prefab_lib.Role.ENTITY,
+        params={
+            "name": "King Charles I",
+            "goal": "avoid execution for treason. " + LANGUAGE_DIRECTIVE.format(lang="French"),
+        },
+    ),
+    prefab_lib.InstanceConfig(
+        prefab="generic__GameMaster",
+        role=prefab_lib.Role.GAME_MASTER,
+        params={
+            "name": "default rules",
+            # Comma-separated list of thought chain steps.
+            "extra_event_resolution_steps": "",
+        },
+    ),
+    prefab_lib.InstanceConfig(
+        prefab="formative_memories_initializer__GameMaster",
+        role=prefab_lib.Role.INITIALIZER,
+        params={
+            "name": "initial setup rules",
+            "next_game_master_name": "default rules",
+            "shared_memories": [
+                "The king was captured by Parliamentary forces in 1646.",
+                "Charles I was tried for treason and found guilty.",
+            ],
+        },
+    ),
+]
+
+config = prefab_lib.Config(
+    default_premise="Today is January 29, 1649.",
+    default_max_steps=5,
+    instances=instances,
+)
+
+
+
+eof