diff --git a/README.md b/README.md
index b512ea7..51f7352 100644
--- a/README.md
+++ b/README.md
@@ -183,8 +183,8 @@ docker-compose -f docker-compose.cuda.yml up -d --build
 | --ga-id                     | Enable Google Analytics on the API client page by providing an ID                                           | `No tracking`               | LT_GA_ID                     |
 | --debug                     | Enable debug environment                                                                                    | `False`           | LT_DEBUG                     |
 | --ssl                       | Whether to enable SSL                                                                                       | `False`               | LT_SSL                       |
-| --frontend-language-source  | Set frontend default language - source                                                                      | `en`          | LT_FRONTEND_LANGUAGE_SOURCE  |
-| --frontend-language-target  | Set frontend default language - target                                                                      | `es`          | LT_FRONTEND_LANGUAGE_TARGET  |
+| --frontend-language-source  | Set frontend default language - source                                                                      | `auto`        | LT_FRONTEND_LANGUAGE_SOURCE  |
+| --frontend-language-target  | Set frontend default language - target                                                                      | `locale` (match site's locale)   | LT_FRONTEND_LANGUAGE_TARGET  |
 | --frontend-timeout          | Set frontend translation timeout                                                                            | `500`         | LT_FRONTEND_TIMEOUT          |
 | --api-keys                  | Enable API keys database for per-user rate limits lookup                                                    | `Don't use API keys` | LT_API_KEYS                  |
 | --api-keys-db-path          | Use a specific path inside the container for the local database. Can be absolute or relative                | `db/api_keys.db`                      | LT_API_KEYS_DB_PATH          |
diff --git a/libretranslate/app.py b/libretranslate/app.py
index 662351d..b29cce5 100644
--- a/libretranslate/app.py
+++ b/libretranslate/app.py
@@ -132,10 +132,33 @@ def create_app(args):
             iter([l for l in languages if l.code == args.frontend_language_source]),
             None,
         )
+    if frontend_argos_language_source is None:
+        frontend_argos_language_source = languages[0]
 
-    frontend_argos_language_target = next(
-        iter([l for l in languages if l.code == args.frontend_language_target]), None
-    )
+    
+    if len(languages) >= 2:
+        language_target_fallback = languages[1]
+    else:
+        language_target_fallback = languages[0]
+
+    if args.frontend_language_target == "locale":
+      def resolve_language_locale():
+          loc = get_locale()
+          language_target = next(
+              iter([l for l in languages if l.code == loc]), None
+          )
+          if language_target is None:
+            language_target = language_target_fallback
+          return language_target
+
+      frontend_argos_language_target = resolve_language_locale
+    else:
+      language_target = next(
+          iter([l for l in languages if l.code == args.frontend_language_target]), None
+      )
+      if language_target is None:
+        language_target = language_target_fallback
+      frontend_argos_language_target = lambda: language_target
 
     frontend_argos_supported_files_format = []
 
@@ -143,15 +166,6 @@ def create_app(args):
         for ff in file_format.supported_file_extensions:
             frontend_argos_supported_files_format.append(ff)
 
-    # Raise AttributeError to prevent app startup if user input is not valid.
-    if frontend_argos_language_source is None:
-        frontend_argos_language_source = languages[0]
-    if frontend_argos_language_target is None:
-        if len(languages) >= 2:
-            frontend_argos_language_target = languages[1]
-        else:
-            frontend_argos_language_target = languages[0]
-
     api_keys_db = None
 
     if args.req_limit > 0 or args.api_keys or args.daily_req_limit > 0:
@@ -896,6 +910,8 @@ def create_app(args):
                           type: string
                           description: Human-readable language name (in English)
         """
+        target_lang = frontend_argos_language_target()
+
         return jsonify(
             {
                 "charLimit": args.char_limit,
@@ -911,8 +927,8 @@ def create_app(args):
                         "name": _lazy(frontend_argos_language_source.name),
                     },
                     "target": {
-                        "code": frontend_argos_language_target.code,
-                        "name": _lazy(frontend_argos_language_target.name),
+                        "code": target_lang.code,
+                        "name": _lazy(target_lang.name),
                     },
                 },
             }
diff --git a/libretranslate/default_values.py b/libretranslate/default_values.py
index c066cbb..e3e9113 100644
--- a/libretranslate/default_values.py
+++ b/libretranslate/default_values.py
@@ -93,12 +93,12 @@ _default_options_objects = [
     },
     {
         'name': 'FRONTEND_LANGUAGE_SOURCE',
-        'default_value': 'en',
+        'default_value': 'auto',
         'value_type': 'str'
     },
     {
         'name': 'FRONTEND_LANGUAGE_TARGET',
-        'default_value': 'es',
+        'default_value': 'locale',
         'value_type': 'str'
     },
     {