Should the Catholic rosary be recited in Latin or the vernacular language? Should the Catholic prayer be recited in Latin or the vernacular language? During recitation, should the Catholic recite by chanting the verses or by singing with a catchy melody? Is it a sin to use the wrong words or switch the order of the verses around? Is it a sin to recite without even understanding the words because the words and concepts might be above the prayer's own vocabulary, or would the young Catholic be taught the meaning and form of the words to avoid praying in vain (Matthew 6:7)?
In the Catholic Church, there are two "types" of worship. On the one hand, there are the public liturgies of the Church (primarily the Mass and the Liturgy of the Hours); on the other hand, there are all other devotions. These are known as private devotions, and, for the most part, are not subject to rules specifying how they are to be prayed.

We see this described in the Vatican II document Sacrosanctum Concilium:


  Popular devotions of the Christian people are to be highly commended,
  provided they accord with the laws and norms of the Church, above all
  when they are ordered by the Apostolic See.
  
  Devotions proper to individual Churches also have a special dignity if
  they are undertaken by mandate of the bishops according to customs or
  books lawfully approved.
  
  But these devotions should be so drawn up that they harmonize with the
  liturgical seasons, accord with the sacred liturgy, are in some
  fashion derived from it, and lead the people to it, since, in fact,
  the liturgy by its very nature far surpasses any of them.


The Rosary is in this second class. In the case of this devotion, there are various guidelines issued, but there are no rules that specify whether or not it is a "valid" Rosary.

It is certainly not a sin to change it according to local or even personal custom, so long as the changes are not contrary to the faith of the Church. It is also up to your personal preference whether you recite it in the vernacular, in Latin, or indeed in Martian if that helped your personal piety. Both are very common.

With regard to your final question, one of the principal advantages of the Rosary is that it is a form of prayer that can be done without complex instruction and is open to various levels of understanding according to the individual's capability.