I agree with the reprimand, I think only the approved translation should be used.
A special degree of flexibility is given to the Lamb of God. From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“83. … The supplication Agnus Dei, is, as a rule, sung by the choir or cantor with the congregation responding; or it is, at least, recited aloud. This invocation accompanies the fraction and, for this reason, may be repeated as many times as necessary until the rite has reached its conclusion, the last time ending with the words
dona nobis pacem (grant us peace).”
It can be done in Latin or with the approved translation.
The 2004 Instruction Redemptionis Sacramentum includes:
“[59.] The reprobated practice by which Priests, Deacons or the faithful here and there alter or vary at will the texts of the Sacred Liturgy that they are charged to pronounce, must cease. For in doing thus, they render the celebration of the Sacred Liturgy unstable, and not infrequently distort the authentic meaning of the Liturgy.”
I see this as forbidding new words in the Lamb of God.
At the Liturgy Office of England and Wales
catholic-ew.org.uk/liturgy/Documents/index.html there is a document that can be downloaded “Roman Missal - a guide for composers”. At the beginning it has that it is a draft document. But it includes, about the Mass texts in general:
“
Adaptation of Texts
2. The repetition of words and phrases within a particular text is permissible but it should neither unduly prolong the setting nor affect the structure or meaning of the text.
3. To facilitate participation refrains for the assembly, based on the text, may be added to texts such as the Gloria.
4. In some cases texts may be adapted slightly to accommodate the musical line (for example the Memorial acclamation “When we eat this bread…” might be expressed “When we eat this living bread…”) but care should be taken that the essential meaning of the text is respected. These adaptations should be based on the text itself, or scriptural or liturgical sources.”
I disagree with all of these, seeing them as a breach of Redemptionis Sacramentum n. 59. Repetition should only be allowed when there is explicit permission to do so, such as for the Lamb of God.
People can be asked to sing different hymns at every Mass. So there is plenty of scope for variety in what they sing. I see it as a problem to be changing the way Mass texts themselves are sung.