BtN has it as a form of Lucia/Lucy used in Wales, but the orthography isn't consistent with the Welsh language. Ll in Welsh is not a long "l", but a voiceless alveolar lateral fricative, a sound common in Native American languages (often transcribed tl or lt) and forming a sound similar to "ch" in English. It doesn't develop naturally from older/foreign words with l, but from earlier sequences of t-l or th-l. Only an English speaker would think
Lucy would be written
Lleucu in Welsh. Since Welsh was suppressed for many generations (less than a quarter of the population of Wales can speak Welsh) it could be common in Wales as a Pseudo-Welsh variant of
Lucy, but it's fake Welsh, neither the actual development of
Lucia in Welsh or another genuine Welsh name.