diff --git a/lobby/i18n.py b/lobby/i18n.py
index 220571f..0a8ac89 100644
--- a/lobby/i18n.py
+++ b/lobby/i18n.py
@@ -36,7 +36,13 @@ def lobby_i18n_error_messages() -> dict:
 
 def resolve_locale(request: HttpRequest) -> str:
     default_locale, supported_locales = i18n_locale_config()
-    requested = (get_language_from_request(request) or "").split("-", 1)[0].lower()
+
+    raw_accept_language = (request.META.get("HTTP_ACCEPT_LANGUAGE") or "").split(",", 1)[0]
+    raw_requested = raw_accept_language.split(";", 1)[0].strip().replace("_", "-").split("-", 1)[0].lower()
+    if raw_requested in supported_locales:
+        return raw_requested
+
+    requested = (get_language_from_request(request) or "").replace("_", "-").split("-", 1)[0].lower()
     if requested in supported_locales:
         return requested
     return default_locale
diff --git a/lobby/tests.py b/lobby/tests.py
index 84e631e..b8ec79d 100644
--- a/lobby/tests.py
+++ b/lobby/tests.py
@@ -1263,6 +1263,17 @@ class I18nResolverTests(TestCase):
         self.assertEqual(response.status_code, 400)
         self.assertEqual(resolve_locale(response.wsgi_request), "da")
 
+    def test_resolve_locale_accepts_underscore_language_tags(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da_DK",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(resolve_locale(response.wsgi_request), "da")
+
     def test_resolve_locale_defaults_to_en_when_header_missing(self):
         response = self.client.post(
             reverse("lobby:join_session"),