diff --git a/backend/internal/account/account.go b/backend/internal/account/account.go
index 43998c9..2324a2c 100644
--- a/backend/internal/account/account.go
+++ b/backend/internal/account/account.go
@@ -534,7 +534,7 @@ const guestDisplayName = "Guest"
 // and history. Guests are not reused — each bootstrap mints a new account. browserTZ
 // (the client's detected "±HH:MM" UTC offset) seeds the guest's time zone, falling
 // back to the 'UTC' default when empty or malformed.
-func (s *Store) ProvisionGuest(ctx context.Context, browserTZ string) (Account, error) {
+func (s *Store) ProvisionGuest(ctx context.Context, browserTZ, language string) (Account, error) {
 	accountID, err := uuid.NewV7()
 	if err != nil {
 		return Account{}, fmt.Errorf("account: new guest id: %w", err)
@@ -543,9 +543,17 @@ func (s *Store) ProvisionGuest(ctx context.Context, browserTZ string) (Account,
 	if tz == "" {
 		tz = "UTC"
 	}
+	// Seed the interface language from the client's detected locale (validated to a supported
+	// one), so a fresh guest's language-dependent server content — the ad banner, bot messages —
+	// is right from first contact rather than the 'en' column default until the client's later
+	// language reconcile catches up. An unsupported or absent code keeps the 'en' default.
+	lang := supportedLanguage(language)
+	if lang == "" {
+		lang = "en"
+	}
 	stmt := table.Accounts.
-		INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.IsGuest, table.Accounts.TimeZone).
-		VALUES(accountID, guestDisplayName, true, tz).
+		INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.IsGuest, table.Accounts.TimeZone, table.Accounts.PreferredLanguage).
+		VALUES(accountID, guestDisplayName, true, tz, lang).
 		RETURNING(table.Accounts.AllColumns)
 
 	var row model.Accounts
diff --git a/backend/internal/inttest/account_test.go b/backend/internal/inttest/account_test.go
index 9e0e1f4..5527741 100644
--- a/backend/internal/inttest/account_test.go
+++ b/backend/internal/inttest/account_test.go
@@ -229,21 +229,36 @@ func TestProvisionSeedsTimeZone(t *testing.T) {
 		t.Errorf("TimeZone = %q, want UTC fallback for a malformed offset", bad.TimeZone)
 	}
 
-	// A guest is seeded its detected offset; an empty one keeps the UTC default.
-	guest, err := store.ProvisionGuest(ctx, "-05:30")
+	// A guest is seeded its detected offset and interface language; an empty offset keeps the
+	// UTC default and an empty/unsupported language keeps the 'en' default.
+	guest, err := store.ProvisionGuest(ctx, "-05:30", "ru")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
 	if guest.TimeZone != "-05:30" {
 		t.Errorf("guest TimeZone = %q, want the seeded -05:30", guest.TimeZone)
 	}
-	plainGuest, err := store.ProvisionGuest(ctx, "")
+	if guest.PreferredLanguage != "ru" {
+		t.Errorf("guest PreferredLanguage = %q, want the seeded ru", guest.PreferredLanguage)
+	}
+	plainGuest, err := store.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision plain guest: %v", err)
 	}
 	if plainGuest.TimeZone != "UTC" {
 		t.Errorf("plain guest TimeZone = %q, want UTC default", plainGuest.TimeZone)
 	}
+	if plainGuest.PreferredLanguage != "en" {
+		t.Errorf("plain guest PreferredLanguage = %q, want en default", plainGuest.PreferredLanguage)
+	}
+	// An unsupported locale falls back to the 'en' default rather than persisting a junk value.
+	frGuest, err := store.ProvisionGuest(ctx, "", "fr-FR")
+	if err != nil {
+		t.Fatalf("provision fr guest: %v", err)
+	}
+	if frGuest.PreferredLanguage != "en" {
+		t.Errorf("unsupported-locale guest PreferredLanguage = %q, want en default", frGuest.PreferredLanguage)
+	}
 }
 
 // TestProvisionTelegramUnknownLanguageDefaults checks an unsupported Telegram
diff --git a/backend/internal/inttest/delete_test.go b/backend/internal/inttest/delete_test.go
index 802e0e4..984321c 100644
--- a/backend/internal/inttest/delete_test.go
+++ b/backend/internal/inttest/delete_test.go
@@ -216,7 +216,7 @@ func TestConfirmCodeClearsGuest(t *testing.T) {
 	mailer := &capturingMailer{}
 	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 
-	guest, err := store.ProvisionGuest(ctx, "")
+	guest, err := store.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
diff --git a/backend/internal/inttest/email_test.go b/backend/internal/inttest/email_test.go
index 9d4235a..2dcd1b3 100644
--- a/backend/internal/inttest/email_test.go
+++ b/backend/internal/inttest/email_test.go
@@ -431,7 +431,7 @@ func TestConfirmByTokenLinkClearsGuest(t *testing.T) {
 	store := account.NewStore(testDB)
 	mailer := &capturingMailer{}
 	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
-	guest, err := store.ProvisionGuest(ctx, "")
+	guest, err := store.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
diff --git a/backend/internal/inttest/helpers.go b/backend/internal/inttest/helpers.go
index 6493cc4..fdf2e60 100644
--- a/backend/internal/inttest/helpers.go
+++ b/backend/internal/inttest/helpers.go
@@ -154,7 +154,7 @@ func provisionAccount(t *testing.T) uuid.UUID {
 // provisionGuest creates a fresh ephemeral guest account and returns its id.
 func provisionGuest(t *testing.T) uuid.UUID {
 	t.Helper()
-	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background(), "")
+	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background(), "", "")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
diff --git a/backend/internal/inttest/userlist_test.go b/backend/internal/inttest/userlist_test.go
index cfb7ac7..2d6df1e 100644
--- a/backend/internal/inttest/userlist_test.go
+++ b/backend/internal/inttest/userlist_test.go
@@ -26,7 +26,7 @@ func TestUserListFilter(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provision robot: %v", err)
 	}
-	guest, err := st.ProvisionGuest(ctx, "")
+	guest, err := st.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
diff --git a/backend/internal/server/handlers_auth.go b/backend/internal/server/handlers_auth.go
index e9e073d..a24c6a6 100644
--- a/backend/internal/server/handlers_auth.go
+++ b/backend/internal/server/handlers_auth.go
@@ -160,16 +160,20 @@ type guestAuthRequest struct {
 	// Subtype is the client-reported device family (ios/android/web) of this direct
 	// (web/native) session; there is no external signer, so it is recorded best-effort.
 	Subtype string `json:"subtype"`
+	// Language is the client's detected interface locale, seeding the fresh guest's
+	// interface language (best-effort; an unsupported/absent value keeps the 'en' default).
+	Language string `json:"language"`
 }
 
 // handleGuestAuth provisions a fresh ephemeral guest account and mints a session,
-// seeding its time zone from the optional detected browser offset.
+// seeding its time zone from the optional detected browser offset and its interface
+// language from the optional detected locale.
 func (s *Server) handleGuestAuth(c *gin.Context) {
-	// The body is optional: an absent or malformed one simply yields no time-zone seed
-	// (the account keeps the UTC default), so a bind error must not fail the bootstrap.
+	// The body is optional: an absent or malformed one simply yields no time-zone/language seed
+	// (the account keeps the UTC / 'en' defaults), so a bind error must not fail the bootstrap.
 	var req guestAuthRequest
 	_ = c.ShouldBindJSON(&req)
-	acc, err := s.accounts.ProvisionGuest(c.Request.Context(), req.BrowserTZ)
+	acc, err := s.accounts.ProvisionGuest(c.Request.Context(), req.BrowserTZ, req.Language)
 	if err != nil {
 		s.abortErr(c, err)
 		return
diff --git a/gateway/internal/backendclient/api.go b/gateway/internal/backendclient/api.go
index 7610e07..e4bf934 100644
--- a/gateway/internal/backendclient/api.go
+++ b/gateway/internal/backendclient/api.go
@@ -312,11 +312,12 @@ func (c *Client) ChatAccessByUser(ctx context.Context, userID string) (ChatAcces
 }
 
 // GuestAuth provisions a guest account and mints a session, seeding its time zone
-// from browserTz (the client's detected "±HH:MM" UTC offset).
-func (c *Client) GuestAuth(ctx context.Context, browserTz, subtype string) (SessionResp, error) {
+// from browserTz (the client's detected "±HH:MM" UTC offset) and its interface
+// language from language (the client's detected locale; best-effort).
+func (c *Client) GuestAuth(ctx context.Context, browserTz, subtype, language string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/guest", "", "",
-		map[string]string{"browser_tz": browserTz, "subtype": subtype}, &out)
+		map[string]string{"browser_tz": browserTz, "subtype": subtype, "language": language}, &out)
 	return out, err
 }
 
diff --git a/gateway/internal/transcode/transcode.go b/gateway/internal/transcode/transcode.go
index 6fc60b5..f030405 100644
--- a/gateway/internal/transcode/transcode.go
+++ b/gateway/internal/transcode/transcode.go
@@ -270,13 +270,14 @@ func authGuestHandler(backend *backendclient.Client) Handler {
 		// The guest bootstrap historically carried no payload; the detected zone is
 		// optional, so an absent or empty one simply yields no time-zone seed (rather
 		// than panicking in GetRootAs* on a zero-length buffer).
-		var browserTz, subtype string
+		var browserTz, subtype, locale string
 		if len(req.Payload) > 0 {
 			g := fb.GetRootAsGuestLoginRequest(req.Payload, 0)
 			browserTz = string(g.BrowserTz())
 			subtype = string(g.Subtype())
+			locale = string(g.Locale())
 		}
-		sess, err := backend.GuestAuth(ctx, browserTz, subtype)
+		sess, err := backend.GuestAuth(ctx, browserTz, subtype, locale)
 		if err != nil {
 			return nil, err
 		}