According to a language-integrated view of spelling development, learning to spell involves the same language-learning skills across alphabetic systems. A prediction based on this view is that the same spelling training should be equally effective for learning to spell in a shallow (Italian, native language) or an opaque (English, additional language) orthography. We tested this prediction by teaching 6- to 9-year-old Italian children to use multiletter spelling units to spell words in Italian and English. The children were trained on the spelling of Italian words containing orthographic difficulties that required switching from phoneme-grapheme spelling correspondences to larger grain size (multiletter) spelling units. In a stepped-wedge cluster randomized trial, 108 Italian children (ages 6-9 years) were assigned to the experimental spelling training or a waiting list condition. Their ability to spell the trained (Italian and English) word lists and to generalize the acquired knowledge to new (untrained) words was assessed. Similar learning effects were found in the two languages for the trained word lists. However, generalization of the acquired spelling knowledge to new words occurred only in English. The influence of language-specific factors on learning to spell could account for these findings.