That said, there are official Roman Catholic liturgy and music books. These are
- The Roman Missal, which has the overall liturgy structures and priest's parts, and
- The Roman Gradual which has Gregorian chant settings of the Mass music intended to be sung by a choir: the standard Gradual has complex chant settings, and the Graduale Simplex has easier settings of the same texts.
The texts in the Graduals are made up of the"ordinary", ie parts that are the same at many different Masses throughout the year) and the "propers", ie short scriptural phrases, generally verses from the Psalms, that are assigned, ie "proper to", individual Masses in the liturgical calendar and used to accompany the opening procession, offertory and communion.
The Roman Missal, Gradual and Gradual Simplex are all in Latin. Vatican II introduced many changes to Catholic liturgy, and therefore to these books:
- There are now translations into the vernacular, ie the living languages (English, Spanish, French, etc) that people use in their daily lives - and now for their common prayer and worship.
- The form of the main Catholic liturgy (the Mass) changed.
- There is an option to use a hymn from an approved collection instead of the Gregorian-chant setting of the "Propers" text.
In practise, people had been singing some local-language hymns at "Low Masses" for some time, but the high/low distinction was removed in the new rite (officially called Novus Ordo) there is simply Mass, and singing hymns became even more common.
Shortly after Vatican II, translations of the Roman Missal into many different languages were prepared - very quickly. The first English translation was in 1973, and a revised English-language translation was published in 2010.
People have been publishing hymnals (ie collections of hymns and songs in various languages for over 400 years. Some hymnals simply have hymns in the main language used in thy country where they were published. But others also contain some Gregorian chants in Latin - as well as hymns and songs from other languages commonly used in the country (eg Irish in Ireland, Welsh in England, Spanish in the USA).
But the liturgical-music changes introduced by Vatican II lead to a far wider variety of hymns being written and hymnals being published in quick succession. It became clear that the idea of substituting a hymn "from an approved collection" was problematic: if a collection is approved this year, then it will not include hymns that are written next year. And the 40 years after Vatican II saw major year-on-year improvements in the standard of hymns which were written for Catholic liturgical use, It wasn't feasible for Catholic bishops to approve new collections every year, and no one believed that it was desirable to keep using lower-quality approved hymns in preference to better ones published in collections that were not-yet-approved.
So in practise, the requirement to use hymns only from "approved collections" was widely ignored. The English-speaking bishops in Canada did publish an official hymnal - and even they say in their current music suggestions guidelines
"Those who use other hymnals are advised to use the titles and themes of the selections suggested to choose music with similar themes from their own repertoire."
Some bishops conferences tried the idea of an approved list of hymns instead of a list of collections. When this was first tried, the technology limitations of the day made it challenging to update people about changes in "the list" . More recently, the Australian and New Zealand bishops have adopted this approach again, with varying degrees of success.
Even today, the hymns books used in English-language Catholic churches are more likely to be widely accepted than officially approved and published having received an imprimatur.