I believe that if you can speak one language well, you can speak many -- that is your proof and you can take it from there. However, the decision to master a new tongue is a brave one. Learning a new language is not picking up few greetings as a tourist. It is a lifetime project. When someone decides to learn or not to learn an x language, it is a worthwhile decision and it is not always right or wrong. That, I believe, is the reason why some people can not learn a new language, they can, but they do not want to.