Dango

Dango: A 1.8B LLM trained exclusively on Japanese to study L1-to-L2 language transfer

Researchers introduce Dango, a 1.8B-parameter decoder-only LLM pretrained strictly on Japanese (L1) and fine-tuned on LLM-generated English (L2) learning lessons to simulate second language acquisition. A key contribution is a filtering method to remove L2 contamination from ostensibly monolingual pretraining corpora. Evaluations show Dango produces human-like L2 error patterns, outperforming multilingual and unfiltered baselines. The model, data, and code are released for computational SLA research.