We have the practice of having the deacon say the words: "Let us proclaim the Mystery of Faith" which comes after the consecration. Is this correct?

The full rubrics for the role of the deacon can be found in the General Instruction, Chapter IV, Part I b. These rubrics do not allow the deacon to say any part of the Eucharistic Prayer. The Eucharistic Prayer is reserved solely to the priest. Any changes such as this upset the natural hierarchical structure of the liturgy and is an abuse. (Inaestimabile donum, n. 4.)

However, we received the following from a visitor in regards to German law:

The German "Messbuch" which is an authentical edition for liturgical use, edited by order of the Episcopal Conferences of Germany, Austria and Switzerland and the bishops of Lusemburg, Bolzano-Brixen and Liège, from 1975 (2nd edition 1988) says: "Dann spricht oder singt er (oder der Diakon: Geheimnis des Glaubens" (English translation: "Then he [the celebrant] (or the deacon) says or sings: This is a mystery of faith". The "Messbuch" has gotten the "confirmatum" from the Congregation of Divine Worship on December 10th, 1974 (prot. no. 2330/74) and July 13, 1987 (prot. no. CD 924/87).