First what is going on with the European true breeding golden duckwings!
These birds are your basic light brown leghorns(e+/e+ s+/s+ co+/co+ mh+/mh+) but with autosomal recessive cream gene(ig/ig) I am not going to go in the the history on how cream ig/ig was introduced in to light brown leghorns but I would like to point out that Professor Punnett received blue egg laying crested Chilean hens from botanist Clarence Elliott in 1929. One of these hens led to Professor Punnett’s monumental discovery of the recessive cream color in poultry in 1931.
So what you need to introduce in your light brown leghorns is the cream gene from a donor with a very similar type to leghorns(just can't find them here in the USA), lucky for you the Cream Legbars have made their way to the USA, now bee in the look out for quality stock as some "cream Legbars" lines have been badly bred and even outcrossed.
here are pics of outstanding quality Cream Legbars
View attachment 1683893
having found a good donor pair, what you need to start doing is crossing your best leghorn rooster to a quality CLB hen, this will produce F1 pullets that are none barred and carry one copy of autosomal recessive cream(Ig+/ig) taking it from there should be really straight forward.