I have this:

\definemode[EN,FR,RU,PT][keep]
\doifmodeelse{EN}
  {
    \disablemode[FR]
    \disablemode[RU]
    \disablemode[PT]
  }
  {
    \doifmodeelse{FR}
    {
      \disablemode[EN]
      \disablemode[RU]
      \disablemode[PT]
    }
    {
      \doifmodeelse{RU}
      {
        \disablemode[EN]
        \disablemode[FR]
        \disablemode[PT]
      }
      {
        \doifmodeelse{PT}
        {
          \disablemode[EN]
          \disablemode[FR]
          \disablemode[RU]
        }
        {
          \enablemode[EN]
        }
      }
    }
  }

But for every language added this becomes more and more complex. I was wondering what would be a good approach to simplify it. 

I want to make sure that one and only one of these modes is active and that if no mode is entered on the command line the mode is EN.