/// <summary>
        /// Add unit to unit list map.
        /// </summary>
        /// <param name="language">Language of the unit.</param>
        /// <param name="type">Type of the unit.</param>
        /// <param name="unit">Unit to be added.</param>
        public void AddUnit(Language language, UnitList.UnitListType type,
            UnitItem unit)
        {
            string unitListKey = UnitList.GetKey(language, type);

            if (!_unitListMap.ContainsKey(unitListKey))
            {
                UnitList unitList = new UnitList();
                unitList.Language = language;
                unitList.UnitType = type;
                _unitListMap.Add(unitListKey, unitList);

                unitList.AddUnit(unit);
            }
            else
            {
                UnitList unitList = _unitListMap[unitListKey];
                unitList.AddUnit(unit);
            }
        }