This is just my opinion, but I think you should include Irish Language songs if the story is close enough to the Child Ballad.

A very interesting statement and one that moves us further away from F.J. Child's statement of intent, given in the title of his tome, English & Scottish Ballads.
All of the ballads Child includes are in some form of language that should be comprehensible to those who can read English even if some are in quite archaic versions of English or in related Scottish forms; Lallans, Doric, etc.
If we were to consider all the recognisable ballad stories that have transferred themselves into other languages then we are making a vast extension of Child's original concept and quite quickly shade into entirely different forms of expression.
Let's just take one example - King Orfeo (Child 19) - The text given is from the singing of a Mr Andrew Coutts of Unst, Shetland by Mr Biot Edmondston. Subsequently another text version came to light - a more complete text, from a Bruce Sutherland of Turf House, North Yell, Shetland in 1865.
The ballad has been collected in the oral tradition twice in Shetland, both from distant ancestors of mine, from John Stickle (by Pat Shuldham Shaw) and Kitty Anderson (by Francis Collinson). Both these versions and the one given in Child have response lines in Norn, the ancient Viking language of Shetland. Interestingly, these responses refer to a hart running through a wood though on last month's really enjoyable stay on Shetland, I saw neither tree nor deer!
This ballad has been collected in in a arc of locations from Northern Norway, through Shetland, the Faroes (quite a few times) and even a suggestion of a fragment from Iceland and only the Shetland versions have any English in them.
King Orfeo is taken to be a real historic character, based on a King of Thrace from the 6th century BC and if we are to consider every version of his story then we have to go back at least as far as Ovid and Virgil and all the countless takes that have been put on the story of Orpheus and Eurydice and all many minstrels' stories, novels, romances, operas etc. that have been based on that.

Perhaps someone would like to undertake that study and tell me they get on!