diff --git a/homeassistant/components/google_assistant_sdk/__init__.py b/homeassistant/components/google_assistant_sdk/__init__.py
index b92b3c54579..4ea496f2824 100644
--- a/homeassistant/components/google_assistant_sdk/__init__.py
+++ b/homeassistant/components/google_assistant_sdk/__init__.py
@@ -26,11 +26,18 @@ from homeassistant.helpers.config_entry_oauth2_flow import (
 )
 from homeassistant.helpers.typing import ConfigType
 
-from .const import DATA_MEM_STORAGE, DATA_SESSION, DOMAIN, SUPPORTED_LANGUAGE_CODES
+from .const import (
+    CONF_LANGUAGE_CODE,
+    DATA_MEM_STORAGE,
+    DATA_SESSION,
+    DOMAIN,
+    SUPPORTED_LANGUAGE_CODES,
+)
 from .helpers import (
     GoogleAssistantSDKAudioView,
     InMemoryStorage,
     async_send_text_commands,
+    best_matching_language_code,
 )
 
 SERVICE_SEND_TEXT_COMMAND = "send_text_command"
@@ -164,9 +171,16 @@ class GoogleAssistantConversationAgent(conversation.AbstractConversationAgent):
         if not session.valid_token:
             await session.async_ensure_token_valid()
             self.assistant = None
-        if not self.assistant or user_input.language != self.language:
+
+        language = best_matching_language_code(
+            self.hass,
+            user_input.language,
+            self.entry.options.get(CONF_LANGUAGE_CODE),
+        )
+
+        if not self.assistant or language != self.language:
             credentials = Credentials(session.token[CONF_ACCESS_TOKEN])  # type: ignore[no-untyped-call]
-            self.language = user_input.language
+            self.language = language
             self.assistant = TextAssistant(credentials, self.language)
 
         resp = await self.hass.async_add_executor_job(
@@ -174,7 +188,7 @@ class GoogleAssistantConversationAgent(conversation.AbstractConversationAgent):
         )
         text_response = resp[0] or "<empty response>"
 
-        intent_response = intent.IntentResponse(language=user_input.language)
+        intent_response = intent.IntentResponse(language=language)
         intent_response.async_set_speech(text_response)
         return conversation.ConversationResult(
             response=intent_response, conversation_id=user_input.conversation_id
diff --git a/homeassistant/components/google_assistant_sdk/helpers.py b/homeassistant/components/google_assistant_sdk/helpers.py
index 24da381e8e0..f9d332cd735 100644
--- a/homeassistant/components/google_assistant_sdk/helpers.py
+++ b/homeassistant/components/google_assistant_sdk/helpers.py
@@ -113,6 +113,33 @@ def default_language_code(hass: HomeAssistant) -> str:
     return DEFAULT_LANGUAGE_CODES.get(hass.config.language, "en-US")
 
 
+def best_matching_language_code(
+    hass: HomeAssistant, assist_language: str, agent_language: str | None = None
+) -> str:
+    """Get the best matching language, based on the preferred assist language and the configured agent language."""
+
+    # Use the assist language if supported
+    if assist_language in SUPPORTED_LANGUAGE_CODES:
+        return assist_language
+    language = assist_language.split("-")[0]
+
+    # Use the agent language if assist and agent start with the same language part
+    if agent_language is not None and agent_language.startswith(language):
+        return best_matching_language_code(hass, agent_language)
+
+    # If assist and agent are not matching, try to find the default language
+    default_language = DEFAULT_LANGUAGE_CODES.get(language)
+    if default_language is not None:
+        return default_language
+
+    # If no default agent is available, use the agent language
+    if agent_language is not None:
+        return best_matching_language_code(hass, agent_language)
+
+    # Fallback to the system default language
+    return default_language_code(hass)
+
+
 class InMemoryStorage:
     """Temporarily store and retrieve data from in memory storage."""
 
diff --git a/tests/components/google_assistant_sdk/test_helpers.py b/tests/components/google_assistant_sdk/test_helpers.py
index 1090eb9da45..4632a86f40f 100644
--- a/tests/components/google_assistant_sdk/test_helpers.py
+++ b/tests/components/google_assistant_sdk/test_helpers.py
@@ -3,6 +3,7 @@
 from homeassistant.components.google_assistant_sdk.const import SUPPORTED_LANGUAGE_CODES
 from homeassistant.components.google_assistant_sdk.helpers import (
     DEFAULT_LANGUAGE_CODES,
+    best_matching_language_code,
     default_language_code,
 )
 from homeassistant.core import HomeAssistant
@@ -46,3 +47,42 @@ def test_default_language_code(hass: HomeAssistant) -> None:
     hass.config.language = "el"
     hass.config.country = "GR"
     assert default_language_code(hass) == "en-US"
+
+
+def test_best_matching_language_code(hass: HomeAssistant) -> None:
+    """Test best_matching_language_code."""
+    hass.config.language = "es"
+    hass.config.country = "MX"
+
+    # Assist Language is supported
+    assert best_matching_language_code(hass, "de-DE", "en-AU") == "de-DE"
+    assert best_matching_language_code(hass, "de-DE") == "de-DE"
+
+    # Assist Language is not supported, but agent language has the same "lang" part, and is supported
+    assert best_matching_language_code(hass, "en", "en-AU") == "en-AU"
+    assert best_matching_language_code(hass, "en-XYZ", "en-AU") == "en-AU"
+    # Assist Language is not supported, but agent language has the same "lang" part, but is not supported
+    assert best_matching_language_code(hass, "en", "en-XYZ") == "en-US"
+    assert best_matching_language_code(hass, "en-XYZ", "en-ABC") == "en-US"
+
+    # Assist Language is not supported, agent is not matching or available, falling back to the default of assist lang
+    assert best_matching_language_code(hass, "de", "en-AU") == "de-DE"
+    assert best_matching_language_code(hass, "de-XYZ", "en-AU") == "de-DE"
+    assert best_matching_language_code(hass, "de") == "de-DE"
+    assert best_matching_language_code(hass, "de-XYZ") == "de-DE"
+
+    # Assist language is not existing at all, agent is supported
+    assert best_matching_language_code(hass, "abc-XYZ", "en-AU") == "en-AU"
+
+    # Assist language is not existing at all, agent is not supported, falling back to the agent default
+    assert best_matching_language_code(hass, "abc-XYZ", "de-XYZ") == "de-DE"
+
+    # Assist language is not existing at all, agent is not existing or available, falling back to system default
+    assert best_matching_language_code(hass, "abc-XYZ", "def-XYZ") == "es-MX"
+    assert best_matching_language_code(hass, "abc-XYZ") == "es-MX"
+
+    # Assist language is not existing at all, agent is not existing or available, system default is not supported
+    hass.config.language = "el"
+    hass.config.country = "GR"
+    assert best_matching_language_code(hass, "abc-XYZ", "def-XYZ") == "en-US"
+    assert best_matching_language_code(hass, "abc-XYZ") == "en-US"