The following particulars regarding the history of the Royal Roussillon Regiment of the French Army were contributed to Vol.1, p. 23 of the Roussillon Gazette by the late Major-General J. C. Young, C.B., Colonel of The Royal Sussex Regiment.
The Royal Roussillon Regiment
The corner of France which lies in the angle of the sea and the mountains on the Spanish frontier at the eastern extremity of the Pyrenees is known on modern maps as the Departément Orientales. Locally, one seldom hears the name, however.
Historically, and in the hearts and on the lips of the people, it is still Roussillon. This name it came by in 1659, when the Treaty of the Pyrenees gave to the French that portion of the Spanish Province of Catalonia lying north of the Pyrenees. Roussillon it remained until the name-changing period following the French Revolution, when the present political divisions, called departéments, replaced many of the old names of provinces in France. In the records of regiments of the French Army, 1659-1900, the history of the Royal Roussillon Regiment is given as follows:-
54th Regiment of Infantry of the Line.
Battle honours emblazoned on the colours:-
Alkamer, 1759. Austerlitz, 1805.
Friedland, 1807. Kabylie, 1857.
Originally raised in 1657 from levies made in Roussillon and in Catalonia, it was known as the Catalan-Mazarin Regiment. In 1661 it became the Royal Catalan Regiment, and in 1667 the Royal Roussillon Regiment. In the year 1791 the territorial title was replaced by a number, and the Roussillon Regiment appears today  in the French Army List as the 54th Regiment of Infantry of the Line.
Further information courtesy of Richard Callaghan (Past Curator, The Military Museum of Sussex)
Lineage of the Royal Roussillon Regiment
By 1792, The Royal Roussillon Regiment was split into 2 demi battalions, in common with the rest of the revolutionary army, possibly in the 1777 army reforms. It recruited in the Roussillon area of southern France as well as Catalonia, during the 17th century the part of the Spanish province of Catalonia north of the Pyrenees was ceeded to France.
The Regiment Royal Roussillon was formed on the 26th May 1657 by Joseph, Baron de Caramany. It was reduced to two Battalions by 1792.
1er Battalion, Regiment d’Infanterie de Ligne Royal Roussillon (Nr 54) [1792 title] became the 46eme Regiment d’Infanterie de Ligne1 in 1804., being disbanded post Waterloo in 1815.
The 2eme Battalion, Regiment d’Infanterie de Ligne Royal Roussillon (Nr 54) [1792 title] became the 21ere Regiment d’Infanterie de Ligne2 in 1804, being further retitled Legion de L’Ardeche in 1815, 4eme Regiment d’Infanterie Legere3 in 1820, and 79eme Regiment d’Infanterie de Ligne4 in 1854.
It will be seen from the above that the 54th Line Regiment, which was formed after 1815, therefore has no connection with the Royal Roussillon, and the 79th line was the only direct descendant unit.