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.
Should the Catholic rosary be recited in Latin or the vernacular language?
In short, it is a personal choice!
Not being part of the Sacred Liturgy of the Church, one has always been quite free to say the rosary in the language of one’s choice. Besides that, fewer and fewer people know Latin these days and the Rosary was originally thought of as the Psalter of the common people, many of whom were illiterate in ages past. Many did not even know Latin!
For many traditional Catholics, the choice of Latin would be most fitting. Even then, I know many traditional Religious who actually say their rosaries in whatever language they desire.
The Rosary developed as we know it during the Middle Ages and as such many popular works of piety were encouraged in the vernacular. The Directory on Popular Piety and the Liturgy (Congregation For Divine Worship and the Discipline of the Sacraments) explains is thus:
- Throughout the middle ages many forms of populuar piety gradually emerged or developed. Many of these have been handed down to our times:
the organization of sacred performances depicting the mysteries celebrated during the liturgical year, especially those surrounding the salvific events of Christ's birth, his passion, death and resurrection;
the participation of the faithful was encouraged by the emergence of poetry in the vernacular which was widely used in popular piety;
as a parallel, or even an alternative to many liturgical expressions, several devotional forms appeared; for example, various forms of Eucharistic adoration served to compensate for the rarity with which Holy Communion was received; in the late middle ages, the rosary tended to substitute for the psalter; among the faithful, the pious exercises of Good Friday became a substitute for the Liturgy proper to that day.
Once again, it does not matter which language is employed when reciting the Rosary. When by myself, I prefer to recite it in Latin. Many friends do not know Latin, so we recite it in English.
Occasionally, I will join up with some friends of the Maronite Rite and pray the Rosary in Aramaic with them, the language Jesus spoke while on earth!
The Rosary may recited, chanted or sung as the individuals group desires. I have chanted it in Latin many times.
In conclusion, it really does not matter what language the Most Holy Rosary is said in. The goal is to meditate (contemplate) on the divine mysteries presented before us in the Rosary.