Olaf Masgnusson can negotiate with the Merseians because he speaks not only Eriau but also two other Merseian languages. Today, I heard of an American actor who learned two Indian languages to make films for Bollywood. I cannot speak Irish, French, Latin or Esperanto despite having studied all four languages at different times.
I think that there are two problems:
a language has to be taught and/or learned properly;
it helps if the learner has some aptitude not only for learning one language in childhood but also for learning others later.
It can be done but not easily by everyone.