diff --git a/coding/string_utf8_multilang.cpp b/coding/string_utf8_multilang.cpp
index 51d22c4d2c..86fe581068 100644
--- a/coding/string_utf8_multilang.cpp
+++ b/coding/string_utf8_multilang.cpp
@@ -193,7 +193,7 @@ void StringUtf8Multilang::AddString(int8_t lang, string const & utf8s)
   {
     size_t const next = GetNextIndex(i);
 
-    if ((m_s[i] & 0x3F) == lang)
+    if ((m_s[i] & kLangCodeMask) == lang)
     {
       ++i;
       m_s.replace(i, next - i, utf8s);
@@ -216,7 +216,7 @@ void StringUtf8Multilang::RemoveString(int8_t lang)
   {
     size_t const next = GetNextIndex(i);
 
-    if ((m_s[i] & 0x3F) == lang)
+    if ((m_s[i] & kLangCodeMask) == lang)
     {
       m_s.erase(i, next - i);
       return;
@@ -238,7 +238,7 @@ bool StringUtf8Multilang::GetString(int8_t lang, string & utf8s) const
   {
     size_t const next = GetNextIndex(i);
 
-    if ((m_s[i] & 0x3F) == lang)
+    if ((m_s[i] & kLangCodeMask) == lang)
     {
       ++i;
       utf8s.assign(m_s.c_str() + i, next - i);
@@ -259,7 +259,7 @@ StringUtf8Multilang::TranslationPositions StringUtf8Multilang::GenerateTranslati
   while (i < sz)
   {
     size_t const next = GetNextIndex(i);
-    int8_t const code = m_s[i] & 0x3F;
+    int8_t const code = m_s[i] & kLangCodeMask;
     if (GetLangByCode(code) != kReservedLang)
       result[code] = Position{i + 1, next - i - 1};
 
@@ -282,7 +282,7 @@ bool StringUtf8Multilang::HasString(int8_t lang) const
 
   for (size_t i = 0; i < m_s.size(); i = GetNextIndex(i))
   {
-    if ((m_s[i] & 0x3F) == lang)
+    if ((m_s[i] & kLangCodeMask) == lang)
       return true;
   }
 
diff --git a/coding/string_utf8_multilang.hpp b/coding/string_utf8_multilang.hpp
index 7f84cab87e..9e663af9e0 100644
--- a/coding/string_utf8_multilang.hpp
+++ b/coding/string_utf8_multilang.hpp
@@ -91,6 +91,10 @@ public:
   /// How many languages we support on indexing stage. See full list in cpp file.
   /// TODO(AlexZ): Review and replace invalid languages by valid ones.
   static int8_t constexpr kMaxSupportedLanguages = 64;
+  // 6 bits language code mask. The language code is encoded with 6 bits that are prepended with
+  // "10".
+  static int8_t constexpr kLangCodeMask = 0x3F;
+  static_assert(kMaxSupportedLanguages == kLangCodeMask + 1, "");
   static char constexpr kReservedLang[] = "reserved";
 
   using Languages = buffer_vector<Lang, kMaxSupportedLanguages>;
@@ -112,19 +116,22 @@ public:
   inline void Clear() { m_s.clear(); }
   inline bool IsEmpty() const { return m_s.empty(); }
 
+  // This method complexity is O(||utf8s||) when adding a new name and O(||m_s|| + ||utf8s||) when
+  // replacing an existing name.
   void AddString(int8_t lang, std::string const & utf8s);
   void AddString(std::string const & lang, std::string const & utf8s)
   {
     int8_t const l = GetLangIndex(lang);
-    if (l >= 0)
+    if (l != kUnsupportedLanguageCode)
       AddString(l, utf8s);
   }
 
+  // This method complexity is O(||m_s||).
   void RemoveString(int8_t lang);
   void RemoveString(std::string const & lang)
   {
     int8_t const l = GetLangIndex(lang);
-    if (l >= 0)
+    if (l != kUnsupportedLanguageCode)
       RemoveString(l);
   }
 
@@ -138,7 +145,7 @@ public:
     while (i < sz)
     {
       size_t const next = GetNextIndex(i);
-      int8_t const code = m_s[i] & 0x3F;
+      int8_t const code = m_s[i] & kLangCodeMask;
       if (GetLangByCode(code) != kReservedLang &&
           wrapper(code, m_s.substr(i + 1, next - i - 1)) == base::ControlFlow::Break)
       {