Specific cognitive and linguistic factors, e.g., phonological memory and vocabulary knowledge to name a few, have been shown to influence spelling development in any native language, as well as among individuals learning an additional one. Spelling acquisition is also dependent on language typology, and may be subject to cross-linguistic transfer based on the level of L1 proficiency (e.g., schooling in L1). The present study examined acquisition of English (L2) spelling by a group of 5th and 6th graders (n = 60) whose L1 (Spanish) continues to be the home language, but whose main language of school instruction is L2. We addressed spelling acquisition based on English Language proficiency levels and L1 reading status (i.e., readers or non-readers in native language). Our results suggest that spelling errors reflected L1 interference or interlingual influences, based on structural differences between English and Spanish. Increased levels of L2 proficiency, however, resulted in more intralingual influences as part of the L2 learning process. Phonological memory was the most significant cognitive element supporting L2 spelling, which concurred with previous research. Reading ability in L1 was another contributor to L2 spelling quality, suggesting cross-linguistic transfer. We discuss the possible continuous role of oral L1 influence in the acquisition of L2 spelling and educational implications.