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arxiv: 2606.19170 · v1 · pith:HAMKX2B3new · submitted 2026-06-17 · 💻 cs.CL

Dango: A Strictly L1-Only Large Language Model for Studying Second Language Acquisition

Pith reviewed 2026-06-26 21:09 UTC · model grok-4.3

classification 💻 cs.CL
keywords second language acquisitionL1-to-L2 transferlarge language modelsJapanese-Englishdata contaminationcomputational SLAmodel filteringdecoder-only models
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The pith

A 1.8B model pretrained only on filtered Japanese data acquires human-like English production after targeted lessons.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper builds Dango to study how Japanese speakers acquire English inside a large decoder model without the usual data leaks. It first removes most English from the Japanese pretraining corpus while leaving a small realistic trace, then adds LLM-generated lessons that mimic classroom input. After this sequence the model produces English that matches patterns seen in human learners and beats both unfiltered and ordinary multilingual baselines. The released model and data are meant to let researchers run controlled experiments on L1-to-L2 transfer at scale.

Core claim

Dango is created by filtering a Japanese corpus to limit premature English exposure, pretraining the 1.8B decoder on the cleaned data, and then fine-tuning on generated L2 lessons; the resulting model exhibits human-like L2 production patterns that outperform unfiltered and standard multilingual baselines.

What carries the argument

The filtering method that reduces L2 contamination in the monolingual pretraining corpus while preserving realistic minimal exposure.

If this is right

  • Dango supplies a controllable simulator for testing specific L1-to-L2 transfer predictions at decoder scale.
  • The same filtering-plus-lesson pipeline can be applied to other language pairs to isolate exposure effects.
  • Releasing the model, data, and code enables direct replication of SLA experiments that were previously limited to smaller models.
  • Human-like error patterns in the fine-tuned model can be compared against learner corpora to validate computational SLA claims.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the filtering step proves robust, similar cleaning could improve other multilingual models that currently suffer from unintended cross-lingual leakage.
  • The approach opens the possibility of running ablation studies that vary only the amount of minimal L2 exposure while holding model size fixed.
  • Learner-facing tools could eventually be built by swapping the lesson generator for real classroom materials and measuring alignment with actual student output.

Load-bearing premise

The filtering method successfully reduces premature L2 exposure in the monolingual pretraining corpus while still preserving realistic minimal exposure.

What would settle it

A direct measurement of English token probability or translation accuracy on the filtered model before any L2 fine-tuning that shows no reduction compared with the unfiltered baseline.

Figures

Figures reproduced from arXiv: 2606.19170 by Fei Cheng, Hirokazu Kiyomaru, Shiho Matta, Takashi Kodama, Yin Jou Huang, Yugo Murawaki.

Figure 1
Figure 1. Figure 1: English contamination examples: severe case [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: An overview of our proposed methodology. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Linguistic proficiency assessments across the L1-pretraining stage. [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Dango and llm-jp-3 Japanese-to-English translation performance when trained and evaluated on different [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Error Rates (raw values) of human and LLMs. [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Contamination: document-level parallel data. [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Contamination: word/phrase-level parallel [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: English appearances that are allowed by the [PITH_FULL_IMAGE:figures/full_fig_p012_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Task category breakdown of llm-jp-eval per [PITH_FULL_IMAGE:figures/full_fig_p012_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Case study of English production after com [PITH_FULL_IMAGE:figures/full_fig_p013_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Prompt used to generate textbook-style L2 learning data. [PITH_FULL_IMAGE:figures/full_fig_p014_11.png] view at source ↗
Figure 13
Figure 13. Figure 13: Prompt format used to evaluate the models [PITH_FULL_IMAGE:figures/full_fig_p014_13.png] view at source ↗
Figure 12
Figure 12. Figure 12: Prompt format used to train Dango and llm [PITH_FULL_IMAGE:figures/full_fig_p014_12.png] view at source ↗
Figure 14
Figure 14. Figure 14: Sentence length distribution of the LLM [PITH_FULL_IMAGE:figures/full_fig_p015_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: System prompt for linguistic feature annotation. We build on the framework of [PITH_FULL_IMAGE:figures/full_fig_p016_15.png] view at source ↗
Figure 17
Figure 17. Figure 17: Case study of Dango’s English production [PITH_FULL_IMAGE:figures/full_fig_p017_17.png] view at source ↗
Figure 16
Figure 16. Figure 16: User prompt for annotating Numbers Agree￾ment. Human L1 JSD of UF JSD of ER BCD of ER Japanese 0.0100 0.0214 0.1491 Korean 0.0136 0.0180 0.1496 Urdu 0.0123 0.0257 0.1778 Mandarin 0.0142 0.0253 0.1430 Cantonese 0.0131 0.0436 0.2140 Thai 0.0162 0.0303 0.1818 Malay 0.0112 0.0404 0.1863 [PITH_FULL_IMAGE:figures/full_fig_p017_16.png] view at source ↗
Figure 19
Figure 19. Figure 19: Error Rate distribution of human and LLMs [PITH_FULL_IMAGE:figures/full_fig_p017_19.png] view at source ↗
Figure 21
Figure 21. Figure 21: L1 knowledge injection user prompt for L2 [PITH_FULL_IMAGE:figures/full_fig_p018_21.png] view at source ↗
Figure 22
Figure 22. Figure 22: Dango and llm-jp-3 when trained and evaluated on different levels of Japanese-English data, showing L2 [PITH_FULL_IMAGE:figures/full_fig_p019_22.png] view at source ↗
Figure 23
Figure 23. Figure 23: L1 knowledge injection system prompt for L2 role-playing. The original prompt is from [PITH_FULL_IMAGE:figures/full_fig_p019_23.png] view at source ↗
Figure 24
Figure 24. Figure 24: Our simplified L1 knowledge-injection prompt for L2 role-playing, designed for llm-jp-3.1-instruct. [PITH_FULL_IMAGE:figures/full_fig_p020_24.png] view at source ↗
read the original abstract

We introduce Dango, a 1.8B-parameter large language model designed for controlled studies of L1-to-L2 (Japanese-to-English) transfer in second language acquisition (SLA). While previous studies have explored SLA in language models, they have predominantly relied on smaller or non-decoder models, limiting their ability to generate open-ended text and reducing their suitability as practical L2 simulators. We identify a key challenge when scaling models to this size: L2 contamination within the "monolingual" pretraining corpus used for L1 acquisition. To address this, we propose a filtering method to reduce premature exposure to English while preserving realistic, minimal exposure. We then fine-tune the model on LLM-generated L2-learning lessons to simulate the L2 acquisition process. Our evaluations confirm that Dango develops human-like L2 production patterns, outperforming both unfiltered and standard multilingual baselines. We release the model, data, and code to facilitate reproducible computational SLA research and learner-facing applications.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper introduces Dango, a 1.8B-parameter decoder-only LLM for controlled studies of Japanese-to-English second language acquisition. It identifies L2 contamination in monolingual pretraining corpora as a scaling challenge, proposes a filtering method to enforce realistic minimal English exposure during L1 pretraining, and then fine-tunes the model on LLM-generated L2 lessons. The central claim is that evaluations demonstrate human-like L2 production patterns, with Dango outperforming both unfiltered and standard multilingual baselines; the model, data, and code are released for reproducibility.

Significance. If the filtering successfully produces an L1-dominant corpus and the performance claims are substantiated with detailed metrics, this work would supply a scalable, open decoder-only model for computational SLA research, enabling controlled transfer experiments that smaller or non-decoder models cannot support. The explicit release of model, data, and code is a concrete strength that directly aids reproducibility and downstream learner-facing applications.

major comments (2)
  1. [Abstract] Abstract and evaluation sections: the claim that 'evaluations confirm that Dango develops human-like L2 production patterns, outperforming both unfiltered and standard multilingual baselines' is asserted without any reported metrics, test sets, baselines, statistical tests, or human-judgment protocols, rendering the central empirical result unverifiable from the provided description.
  2. [Methods (filtering procedure)] Methods section on corpus filtering: the proposed filtering procedure is described as the sole mechanism for reducing premature L2 exposure while preserving 'realistic, minimal exposure,' yet no quantitative validation (residual English n-gram rates, ablation on filter aggressiveness, or comparison against actual Japanese child-directed corpora) is supplied; this leaves the load-bearing precondition for attributing later L2 transfer to controlled SLA unanchored.
minor comments (1)
  1. [Introduction] Notation for the 1.8B model size and the distinction between 'unfiltered' and 'standard multilingual' baselines should be defined explicitly on first use to avoid ambiguity for readers outside the immediate SLA subfield.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful review and constructive comments. We address each major comment below and plan to incorporate revisions to strengthen the paper.

read point-by-point responses
  1. Referee: [Abstract] Abstract and evaluation sections: the claim that 'evaluations confirm that Dango develops human-like L2 production patterns, outperforming both unfiltered and standard multilingual baselines' is asserted without any reported metrics, test sets, baselines, statistical tests, or human-judgment protocols, rendering the central empirical result unverifiable from the provided description.

    Authors: We agree with the referee that the abstract and evaluation sections should provide the supporting metrics, test sets, baselines, statistical tests, and human-judgment protocols to substantiate the central claim. We will revise the manuscript to include these details, ensuring the empirical results are fully verifiable. revision: yes

  2. Referee: [Methods (filtering procedure)] Methods section on corpus filtering: the proposed filtering procedure is described as the sole mechanism for reducing premature L2 exposure while preserving 'realistic, minimal exposure,' yet no quantitative validation (residual English n-gram rates, ablation on filter aggressiveness, or comparison against actual Japanese child-directed corpora) is supplied; this leaves the load-bearing precondition for attributing later L2 transfer to controlled SLA unanchored.

    Authors: We agree that quantitative validation is essential for the filtering procedure. We will supplement the methods section with residual English n-gram rates, ablations on filter aggressiveness, and comparisons against actual Japanese child-directed corpora. revision: yes

Circularity Check

0 steps flagged

No circularity; sequential training and evaluation steps remain independent

full rationale

The paper describes a linear pipeline: propose and apply an L2 filter to a Japanese corpus, pretrain the 1.8B model, fine-tune on generated L2 lessons, then run separate evaluations against unfiltered and multilingual baselines. No equations, fitted parameters renamed as predictions, or self-citations are invoked to derive the human-like L2 patterns; the outcome is presented as an empirical result of the procedure rather than a definitional or load-bearing reduction to the filter itself. The filtering step is an assumption whose effectiveness is not independently verified in the provided text, but that is a question of evidence strength, not circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no explicit free parameters, axioms, or invented entities; all details on training objectives, filtering thresholds, or evaluation metrics are absent.

pith-pipeline@v0.9.1-grok · 5721 in / 1090 out tokens · 32743 ms · 2026-06-26T21:09:07.981806+00:00 · methodology

discussion (0)

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