code: def tt(self, key, locale=None, locale2=None, default=I18n.DFT):
          
          
          #/
          locale = locale or self.locale
          
          locale2 = locale2 or self.locale2
          
          #/ get tt dict of the locale
          tt_d = self.tt_dd.get(locale, None)
          
          if tt_d is not None:
              #/
              val = tt_d.get(key, I18n.DFT)
              
              #/ if tt value is found
              if val is not I18n.DFT:
                  return val
          
          #/ tt value is not found
          
          #/ if has locale2
          ## Y
          if locale2 and locale2 != locale:
              #/ fall back to locale2
              return self.tt(key, locale=locale2, default=default)
          ## N
          else:
              #/ if default is specified
              ## N
              if default is I18n.DFT:
                  raise KeyError(key)
              ## Y
              else:
                  return default
docs: |tt| means text transform.
          
          key: tt key.
          locale: main locale key into |self.tt_dd|. Default to |self.locale|
          locale2: fallback locale key into |self.tt_dd|. Default to |self.locale2|
          default: a default value in case tt value is not found. Default to raise KeyError.