diff --git a/td/generate/scheme/td_api.tl b/td/generate/scheme/td_api.tl
index c43fde9b3..2ff9d6bef 100644
--- a/td/generate/scheme/td_api.tl
+++ b/td/generate/scheme/td_api.tl
@@ -3380,16 +3380,19 @@ getLocalizationTargetInfo only_local:Bool = LocalizationTargetInfo;
 //@description Returns strings from a language pack in the current localization target by their keys @language_pack_id Language pack identifier of the strings to be returned @keys Language pack keys of the strings to be returned; leave empty to request all available strings
 getLanguagePackStrings language_pack_id:string keys:vector<string> = LanguagePackStrings;
 
-//@description Adds or changes a custom language pack to the current localization target @info Information about the language pack. Language pack ID must start with 'X', consist only of English letters, digits and hyphens, and must not exceed 64 characters @strings Strings of the new language pack
+//@description Adds a custom server language pack to the current localization target @language_pack_id Identifier of a language pack to be added; may be different from a name that is used in an "https://t.me/setlanguage/" link
+addCustomServerLanguagePack language_pack_id:string = Ok;
+
+//@description Adds or changes a custom local language pack to the current localization target @info Information about the language pack. Language pack ID must start with 'X', consist only of English letters, digits and hyphens, and must not exceed 64 characters @strings Strings of the new language pack
 setCustomLanguagePack info:languagePackInfo strings:vector<languagePackString> = Ok;
 
-//@description Edits information about a custom language pack in the current localization target @info New information about the custom language pack
+//@description Edits information about a custom local language pack in the current localization target @info New information about the custom local language pack
 editCustomLanguagePackInfo info:languagePackInfo = Ok;
 
-//@description Adds, edits or deletes a string in a custom language pack @language_pack_id Identifier of a previously added custom language pack in the current localization target @new_string New language pack string
+//@description Adds, edits or deletes a string in a custom local language pack @language_pack_id Identifier of a previously added custom local language pack in the current localization target @new_string New language pack string
 setCustomLanguagePackString language_pack_id:string new_string:languagePackString = Ok;
 
-//@description Deletes all information about a language pack in the current localization target. The language pack that is currently in use can't be deleted @language_pack_id Identifier of the language pack to delete
+//@description Deletes all information about a language pack in the current localization target. The language pack that is currently in use (including base language pack) can't be deleted @language_pack_id Identifier of the language pack to delete
 deleteLanguagePack language_pack_id:string = Ok;
 
 
diff --git a/td/generate/scheme/td_api.tlo b/td/generate/scheme/td_api.tlo
index 4a3fbf8c5..8e3e18d81 100644
Binary files a/td/generate/scheme/td_api.tlo and b/td/generate/scheme/td_api.tlo differ
diff --git a/td/telegram/LanguagePackManager.cpp b/td/telegram/LanguagePackManager.cpp
index 306aaad1a..0f4ad7525 100644
--- a/td/telegram/LanguagePackManager.cpp
+++ b/td/telegram/LanguagePackManager.cpp
@@ -83,6 +83,7 @@ struct LanguagePackManager::LanguagePack {
   SqliteKeyValue pack_kv_;                                              // usages should be guarded by database_->mutex_
   std::map<string, LanguageInfo> custom_language_pack_infos_;           // sorted by language_code
   vector<std::pair<string, LanguageInfo>> server_language_pack_infos_;  // sorted by server
+  std::unordered_map<string, LanguageInfo> all_server_language_pack_infos_;
   std::unordered_map<string, unique_ptr<Language>> languages_;
 };
 
@@ -465,6 +466,7 @@ LanguagePackManager::Language *LanguagePackManager::add_language(LanguageDatabas
               info.total_string_count_ = to_integer<int32>(all_infos[i + 8]);
               info.translated_string_count_ = to_integer<int32>(all_infos[i + 9]);
               info.translation_url_ = std::move(all_infos[i + 10]);
+              pack->all_server_language_pack_infos_.emplace(all_infos[i], info);
               pack->server_language_pack_infos_.emplace_back(std::move(all_infos[i]), std::move(info));
             }
           } else {
@@ -758,6 +760,7 @@ void LanguagePackManager::on_get_languages(vector<tl_object_ptr<telegram_api::la
     auto pack_it = database_->language_packs_.find(language_pack);
     if (pack_it != database_->language_packs_.end()) {
       LanguagePack *pack = pack_it->second.get();
+      std::lock_guard<std::mutex> pack_lock(pack->mutex_);
       for (auto &info : pack->custom_language_pack_infos_) {
         add_language_info(info.first, info.second);
       }
@@ -845,7 +848,11 @@ void LanguagePackManager::on_get_languages(vector<tl_object_ptr<telegram_api::la
     auto pack_it = database_->language_packs_.find(language_pack);
     if (pack_it != database_->language_packs_.end()) {
       LanguagePack *pack = pack_it->second.get();
+      std::lock_guard<std::mutex> pack_lock(pack->mutex_);
       if (pack->server_language_pack_infos_ != all_server_infos) {
+        for (auto &info : all_server_infos) {
+          pack->all_server_language_pack_infos_.emplace(info.first, info.second);
+        }
         pack->server_language_pack_infos_ = std::move(all_server_infos);
 
         if (!pack->pack_kv_.empty()) {
@@ -1266,6 +1273,40 @@ void LanguagePackManager::on_failed_get_difference(string language_pack, string
   }
 }
 
+void LanguagePackManager::add_custom_server_language(string language_code, Promise<Unit> &&promise) {
+  if (language_pack_.empty()) {
+    return promise.set_error(Status::Error(400, "Option \"localization_target\" needs to be set first"));
+  }
+  if (!check_language_code_name(language_code)) {
+    return promise.set_error(Status::Error(400, "Language pack ID must contain only letters, digits and hyphen"));
+  }
+  if (is_custom_language_code(language_code)) {
+    return promise.set_error(
+        Status::Error(400, "Custom local language pack can't be added through addCustomServerLanguagePack"));
+  }
+
+  if (get_language(database_, language_pack_, language_code) == nullptr) {
+    return promise.set_error(Status::Error(400, "Language pack not found"));
+  }
+
+  std::lock_guard<std::mutex> packs_lock(database_->mutex_);
+  auto pack_it = database_->language_packs_.find(language_pack_);
+  CHECK(pack_it != database_->language_packs_.end());
+  LanguagePack *pack = pack_it->second.get();
+  std::lock_guard<std::mutex> pack_lock(pack->mutex_);
+  auto it = pack->all_server_language_pack_infos_.find(language_code);
+  if (it == pack->all_server_language_pack_infos_.end()) {
+    return promise.set_error(Status::Error(400, "Language pack info not found"));
+  }
+  auto &info = pack->custom_language_pack_infos_[language_code];
+  info = it->second;
+  if (!pack->pack_kv_.empty()) {
+    pack->pack_kv_.set(language_code, get_language_info_string(info));
+  }
+
+  promise.set_value(Unit());
+}
+
 Result<tl_object_ptr<telegram_api::LangPackString>> LanguagePackManager::convert_to_telegram_api(
     tl_object_ptr<td_api::languagePackString> &&str) {
   if (str == nullptr) {
@@ -1398,6 +1439,7 @@ void LanguagePackManager::set_custom_language(td_api::object_ptr<td_api::languag
   auto pack_it = database_->language_packs_.find(language_pack_);
   CHECK(pack_it != database_->language_packs_.end());
   LanguagePack *pack = pack_it->second.get();
+  std::lock_guard<std::mutex> pack_lock(pack->mutex_);
   auto &info = pack->custom_language_pack_infos_[language_code];
   info = r_info.move_as_ok();
   if (!pack->pack_kv_.empty()) {
@@ -1426,6 +1468,7 @@ void LanguagePackManager::edit_custom_language_info(td_api::object_ptr<td_api::l
   auto pack_it = database_->language_packs_.find(language_pack_);
   CHECK(pack_it != database_->language_packs_.end());
   LanguagePack *pack = pack_it->second.get();
+  std::lock_guard<std::mutex> pack_lock(pack->mutex_);
   auto language_info_it = pack->custom_language_pack_infos_.find(language_code);
   if (language_info_it == pack->custom_language_pack_infos_.end()) {
     return promise.set_error(Status::Error(400, "Custom language pack is not found"));
diff --git a/td/telegram/LanguagePackManager.h b/td/telegram/LanguagePackManager.h
index dd7e58e87..668fd3b70 100644
--- a/td/telegram/LanguagePackManager.h
+++ b/td/telegram/LanguagePackManager.h
@@ -61,6 +61,8 @@ class LanguagePackManager : public NetQueryCallback {
 
   void on_update_language_pack(tl_object_ptr<telegram_api::langPackDifference> difference);
 
+  void add_custom_server_language(string language_code, Promise<Unit> &&promise);
+
   void set_custom_language(td_api::object_ptr<td_api::languagePackInfo> &&language_pack_info,
                            vector<tl_object_ptr<td_api::languagePackString>> strings, Promise<Unit> &&promise);
 
diff --git a/td/telegram/Td.cpp b/td/telegram/Td.cpp
index 6746d8d90..1eea195d3 100644
--- a/td/telegram/Td.cpp
+++ b/td/telegram/Td.cpp
@@ -6194,6 +6194,14 @@ void Td::on_request(uint64 id, td_api::getLanguagePackStrings &request) {
                std::move(request.language_pack_id_), std::move(request.keys_), std::move(promise));
 }
 
+void Td::on_request(uint64 id, td_api::addCustomServerLanguagePack &request) {
+  CHECK_IS_USER();
+  CLEAN_INPUT_STRING(request.language_pack_id_);
+  CREATE_OK_REQUEST_PROMISE();
+  send_closure(language_pack_manager_, &LanguagePackManager::add_custom_server_language,
+               std::move(request.language_pack_id_), std::move(promise));
+}
+
 void Td::on_request(uint64 id, td_api::setCustomLanguagePack &request) {
   CHECK_IS_USER();
   CREATE_OK_REQUEST_PROMISE();
diff --git a/td/telegram/Td.h b/td/telegram/Td.h
index 29196b847..9fb24cbf2 100644
--- a/td/telegram/Td.h
+++ b/td/telegram/Td.h
@@ -809,6 +809,8 @@ class Td final : public NetQueryCallback {
 
   void on_request(uint64 id, td_api::getLanguagePackStrings &request);
 
+  void on_request(uint64 id, td_api::addCustomServerLanguagePack &request);
+
   void on_request(uint64 id, td_api::setCustomLanguagePack &request);
 
   void on_request(uint64 id, td_api::editCustomLanguagePackInfo &request);
diff --git a/td/telegram/cli.cpp b/td/telegram/cli.cpp
index b509b124f..6b7cbf686 100644
--- a/td/telegram/cli.cpp
+++ b/td/telegram/cli.cpp
@@ -1846,6 +1846,9 @@ class CliClient final : public Actor {
       std::tie(language_code, key) = split(args);
       send_request(
           make_tl_object<td_api::getLanguagePackString>(language_database_path, language_pack, language_code, key));
+    } else if (op == "acslp") {
+      string language_code = args;
+      send_request(td_api::make_object<td_api::addCustomServerLanguagePack>(language_code));
     } else if (op == "sclp") {
       string language_code;
       string name;