message: util: Enhance bilingual_str
diff: struct bilingual_str {
  std::string original;
  std::string translated;
  +
  + bilingual_str& operator+=(const bilingual_str& rhs)
  + {
  + original += rhs.original;
  + translated += rhs.translated;
  + return *this;
  + }
  };
  -inline bilingual_str operator+(const bilingual_str& lhs, const bilingual_str& rhs)
  +inline bilingual_str operator+(bilingual_str lhs, const bilingual_str& rhs)
  {
  - return bilingual_str{
  - lhs.original + rhs.original,
  - lhs.translated + rhs.translated};
  + lhs += rhs;
  + return lhs;
  }
  /** Mark a bilingual_str as untranslated */
  