mtproto,rpc: support Android legacy startup

(cherry picked from commit 06b6ae2bb2f90bd7cc1d6a8404a82aff507e6821)

internal/rpc/langpack.go

@@ -2,12 +2,19 @@ package rpc
 
 import (
 	"context"
+	"strings"
 
+	"github.com/gotd/td/bin"
 	"github.com/gotd/td/tg"
 )
 
+const legacyLangpackGetLanguagesTypeID uint32 = 0x800fd57d
+
 // registerLangpack 注册 langpack.* RPC handler。
 func (r *Router) registerLangpack(d *tg.ServerDispatcher) {
+	d.OnLangpackGetLanguages(func(ctx context.Context, langPack string) ([]tg.LangPackLanguage, error) {
+		return r.langpackLanguages(ctx, langPack), nil
+	})
 	d.OnLangpackGetLangPack(func(ctx context.Context, req *tg.LangpackGetLangPackRequest) (*tg.LangPackDifference, error) {
 		if r.deps.LangPack == nil {
 			return &tg.LangPackDifference{LangCode: req.LangCode}, nil
@@ -39,3 +46,54 @@ func (r *Router) registerLangpack(d *tg.ServerDispatcher) {
 		return tgLangPackStrings(pack.Strings), nil
 	})
 }
+
+func (r *Router) handleLegacyLangpackGetLanguages(ctx context.Context, b *bin.Buffer) (bin.Encoder, error) {
+	if err := b.ConsumeID(legacyLangpackGetLanguagesTypeID); err != nil {
+		return nil, err
+	}
+	return &tg.LangPackLanguageVector{Elems: r.langpackLanguages(ctx, "")}, nil
+}
+
+func (r *Router) langpackLanguages(ctx context.Context, langPack string) []tg.LangPackLanguage {
+	if langPack == "" {
+		langPack = langPackFromClient(ctx)
+	}
+	_ = langPack
+	return []tg.LangPackLanguage{
+		{
+			Official:        true,
+			Name:            "English",
+			NativeName:      "English",
+			LangCode:        "en",
+			PluralCode:      "en",
+			StringsCount:    0,
+			TranslatedCount: 0,
+			TranslationsURL: "",
+		},
+		{
+			Official:        true,
+			Name:            "Chinese (Simplified)",
+			NativeName:      "Chinese (Simplified)",
+			LangCode:        "zh-hans",
+			PluralCode:      "zh",
+			StringsCount:    0,
+			TranslatedCount: 0,
+			TranslationsURL: "",
+		},
+	}
+}
+
+func langPackFromClient(ctx context.Context) string {
+	info, ok := ClientInfoFrom(ctx)
+	if !ok {
+		return "tdesktop"
+	}
+	if info.LangPack != "" {
+		return info.LangPack
+	}
+	client := strings.ToLower(info.DeviceModel + " " + info.SystemVersion + " " + info.AppVersion)
+	if strings.Contains(client, "android") {
+		return "android"
+	}
+	return "tdesktop"
+}