The actual requirement to say the liturgy of another rite is to be granted faculties OR be a concelebrant.
A roman priest, provided he has roman faculties, may celebrate the Ruthenian DL. To do so as primary celebrant, however, would require him to have been trained in saying the DL and granted faculties.
To do so in the other rite’s vestments, even as a concelebrant, requires formal faculties.
All the following references are to the CCEO; orchid text is quoted directly.
Canon 343 specifies that seminarians must be formed in their own rite (but this may be done in a seminary of another church sui iuris)
Canon 361 allows transfers to eparchies (dioceses) in severe need even across churches sui iuris.
Clerics of whatever Church sui iuris linked as brothers in the bond of charity to attain the unity of all, work together for the building up of the body of Christ and consequently whatever their condition, are to cooperate among themselves and help one another, even performing diverse functions.
Clerics, whatever their condition, are to care in their heart for all of the Churches, and therefore to serve her wherever there is great necessity; let them show willingness, especially with the permission or encouragement of their own eparchial bishop or superior, to exercise their ministry in the missions or in regions laboring under a shortage of clergy.
- In celebrating the sacraments that which is contained in the liturgical books is to be observed accurately.
- The minister should celebrate the sacraments according to the liturgical prescriptions of his own Church sui iuris, unless the law establishes otherwise or he himself has obtained a special faculty from the Apostolic See.
A concelebration between bishops and presbyters of different Churches sui iuris for a just cause, especially that of fostering charity, and for the sake of manifesting unity between the Churches, can be done with the permission of the eparchial bishop, while observing all the prescriptions of the liturgical books of the principal celebrant, having removed any liturgical syncretism and wearing the appropriate vestments and insignia of his own Church sui iuris.
Catholic priests are forbidden to concelebrate the Divine Liturgy with non-Catholic priests or ministers.
- A Catholic priest can celebrate the Divine Liturgy on the altar of any Catholic church.
- In order for a priest to be able to celebrate the Divine Liturgy in a non-Catholic church, he needs the permission of the local hierarch.