Earlier than WW1 but the Royal Navy was notoriously unfussy about nationality and several of the sailors at Trafalgar were French. The Army also used to have no trouble with foreign recruits. One of the 24th Regiment of Foot at Rourkes Drift was Swiss and there was a company of Zulus in the Natal Native Contingent on the British side there too. I believe that even today about 10% of the British Army are foreigners. I recall many Irish soldiers serving during the Troubles and one of my Regimental Police had served with the Irish Army in the Congo. There is a whole history of foreign recruits, foreign troops in British Pay and foreign recruited Army regiments such as the 1st Chinese Regiment raised in Wei Hai Wei or the Colonial Marines Light Infantry raised in the SE USA @1814. The list seems endless. Corsican, Greek, Swiss, Spanish, French................................ In the old days one only needed to be prepared to take the Oath of Attestation. My old Intelligence Section included soldiers of assorted origins: French, Bulgarian, Czech, Indian, Irish. Even a token Englishman or two. As an aside in Italy in WW2 there was an active trade in Polish speaking German POWs being unofficially handed over to the Polish Army in exchange for goodies and instantly ceasing to be bad German POWs and reclothed in British kit as good Polish soldiers. A series of somewhat OT digressions but it sets a flavour of how things used to be done.