mtproto,rpc: support Android legacy startup

(cherry picked from commit 06b6ae2bb2f90bd7cc1d6a8404a82aff507e6821)

internal/rpc/langpack_compat_test.go

@@ -0,0 +1,54 @@
+package rpc
+
+import (
+	"context"
+	"testing"
+
+	"go.uber.org/zap/zaptest"
+
+	"github.com/gotd/td/bin"
+	"github.com/gotd/td/clock"
+	"github.com/gotd/td/tg"
+)
+
+func TestLangpackGetLanguagesCurrentAndLegacy(t *testing.T) {
+	r := New(Config{DC: 2, IP: "127.0.0.1", Port: 2398}, Deps{}, zaptest.NewLogger(t), clock.System)
+
+	t.Run("current layer", func(t *testing.T) {
+		var in bin.Buffer
+		if err := (&tg.LangpackGetLanguagesRequest{LangPack: "tdesktop"}).Encode(&in); err != nil {
+			t.Fatalf("encode request: %v", err)
+		}
+		assertLangpackLanguages(t, r, context.Background(), &in)
+	})
+
+	t.Run("legacy android no args", func(t *testing.T) {
+		var in bin.Buffer
+		in.PutID(legacyLangpackGetLanguagesTypeID)
+		ctx := WithClientInfo(context.Background(), ClientInfo{
+			DeviceModel: "Android",
+			AppVersion:  "12.7.3",
+			LangCode:    "en",
+		})
+		assertLangpackLanguages(t, r, ctx, &in)
+	})
+}
+
+func assertLangpackLanguages(t *testing.T, r *Router, ctx context.Context, in *bin.Buffer) {
+	t.Helper()
+	enc, err := r.Dispatch(ctx, [8]byte{}, 0, in)
+	if err != nil {
+		t.Fatalf("dispatch langpack.getLanguages: %v", err)
+	}
+	var out bin.Buffer
+	if err := enc.Encode(&out); err != nil {
+		t.Fatalf("encode response: %v", err)
+	}
+	var langs tg.LangPackLanguageVector
+	if err := langs.Decode(&out); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+	if len(langs.Elems) == 0 || langs.Elems[0].LangCode != "en" {
+		t.Fatalf("languages = %+v, want English entry", langs.Elems)
+	}
+}