REVIEW 4 major objections 5 minor 36 references
Cross-Lingual Optimization for Language Transfer in Large Language Models
T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read CLO, a modified DPO loss that contrasts English and translated target-language responses in the same batch, improves target-language instruction following and preserves English better than standard SFT.
desk verdict Useful empirical recipe for cross-lingual transfer, but the main loss is just DPO on translated pairs and the attention-only mask is never controlled in the baselines. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
The authors propose Cross-Lingual Optimization (CLO). They start with English instruction-response pairs and translate both the instruction and the response into a target language using a translation model. Then they construct preference pairs: for an English prompt, the English response is marked as good and the translated response as bad; for a translated prompt, the translated response is good and the English response is bad. A preference loss, in the style of Direct Preference Optimization (DPO), trains the model to prefer the response that matches the input language. The model is also trained with a standard language-modeling loss on the target-language responses only.
CLO is tested on five base models (Llama-2, Llama-3, Mistral, Qwen) and six languages ranging from high-resource (Chinese, German) to low-resource (Swahili, Yoruba). The method typically beats standard fine-tuning on instruction-following win rates, reading comprehension (BELEBELE), and reasoning (MMMLU), while also maintaining or improving English scores. The largest gains appear in low-resource languages: for Swahili, CLO trained on about 3,200 examples performs around the level of standard fine-tuning trained on 6,400 examples.
Extended reading notes
Core claim
The central claim is that Cross-Lingual Optimization (CLO), a DPO-based loss with cross-lingual preference pairs and attention-only fine-tuning, 'consistently outperforms SFT in both acquiring target language proficiency and maintaining English performance,' with the strongest gains in low-resource languages where CLO with 3,200 samples surpasses SFT with 6,400 samples (Abstract, Section 6, Table 1). If true, CLO offers a data- and compute-efficient alternative to standard supervised fine-tuning for language transfer.
Load-bearing premise
The paper's first key hypothesis (Section 3): 'given an input query in a non-English target language, suppressing English responses while strengthening responses in the target language enables the model to leverage its existing English knowledge to generate outputs in the target language.' This assumes the model's knowledge is language-independent and can be expressed in any target language once the output-language bias is corrected. If false, CLO would increase language-matching but not real capability, and the BELEBELE/MMMLU gains would vanish. The evidence for transferability is indirect: a single Swahili example and benchmark improvements.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes Cross-Lingual Optimization (CLO), a fine-tuning method for transferring English-centric LLMs to target languages. CLO builds a cross-lingual dataset by translating English instruction data into the target language and defines a loss that combines a target-language Negative Log-Likelihood term with DPO-style preference pairs that suppress English responses to target-language prompts and target-language responses to English prompts. The method also restricts parameter updates to attention layers. Experiments on five LLMs and six languages (Chinese, German, Korean, Indonesian, Swahili, Yoruba) report that CLO achieves higher AlpacaEval win rates against an SFT baseline than SFT+DPO, with additional gains on BELEBELE and MMMLU, and shows better data efficiency in low-resource languages.
Significance. If the results are substantiated, CLO would provide a practical, data-efficient alternative to supervised fine-tuning for language transfer, using only translated English instruction data while preserving English performance. The paper has notable strengths: a broad empirical matrix (five base models across six languages), evaluation on external benchmarks (BELEBELE, MMMLU) that go beyond the custom AlpacaEval setup, and ablation studies that probe the parameter mask and the NLL loss. The main weaknesses are that the causal role of the cross-lingual objective is not isolated from the attention-only mask and that the mathematical novelty of the loss relative to standard DPO is not established. The overall contribution is potentially useful but is currently overstated.
major comments (4)
- [Section 3.2/4.1, Tables 1 and 4] The reported gains of CLO over SFT and SFT+DPO cannot be attributed to the cross-lingual loss because the baselines are not trained with the same attention-only parameter mask. The paper only states that 'all baselines and models' use identical hyperparameters (Section 4.1), which does not cover the parameter mask; standard SFT and DPO implementations update all parameters. Table 4 shows the mask is not neutral: in Swahili, attention-only CLO wins only 29.69% against full-parameter CLO. Without an SFT baseline and an SFT+DPO baseline using the same mask, the consistent win rates in Table 1 could be due to the regularizing effect of parameter-efficient fine-tuning rather than the cross-lingual preference objective. This confound is load-bearing for the paper's central claim that the CLO objective is responsible for the gains.
- [Appendix A, Eqs. (5)-(7), (10)-(12)] The paper claims that L_CL differs from standard DPO by 'utilizing cross-lingual data pairs within the same batch.' However, the loss is an additive average of per-example DPO log-sigmoid terms for English and target preference pairs, with no cross-sample or batch-level coupling. This is mathematically identical to applying the standard DPO loss to the union of the two preference datasets with the same reference model and β. The only remaining differences are the target-only NLL term and the attention mask, not the preference loss itself. The statement in Appendix A that standard DPO 'might not utilize the relationships between accepted and rejected responses across languages' is therefore unsupported by the loss formulation.
- [Table 1] The claim that CLO 'consistently outperforms SFT' is not statistically supported in many cells. For example, Mistral-7B shows target win rates of 50.8±1.76 (German), 50.5±1.76 (Korean), 51.1±1.76 (Indonesian), and 51.1±1.52 (Yoruba), which are within one standard deviation of 50%. Similarly, Llama-2-7B has deltas of +0.4 to +1.3 in Korean, Indonesian, and Swahili. With the reported standard deviations, these differences are not meaningful. The paper should provide significance tests or confidence intervals for the win-rate comparisons, or soften the 'consistent' claim to a directional trend.
- [Section 7.1 and Table 4] The opening statement that attention-only training 'maintains or even improves' performance is contradicted by the table itself, which shows attention-only CLO loses to full-parameter CLO in Swahili by 29.69% win versus 69.57% lose. While the next sentence acknowledges this drop, the blanket claim should be revised to state the limitation, especially because Swahili is one of the headline low-resource languages in the abstract.
minor comments (5)
- [Abstract and Section 6] The abstract states 'CLO with only 3,200 samples surpasses SFT with 6,400 samples,' while Section 6 describes the 1,600-sample CLO as 'comparable' to the 6,400-sample SFT and the 3,200-sample CLO as 'superior.' Please align the wording for consistency.
- [Section 5, BELEBELE] There is a typo: 'BELEBLE' should be 'BELEBELE'.
- [Appendix D] The description of the answer-extraction procedure ('the evaluation model included 10 English and 10 target language test samples per target language in its training') is unclear; specify which model is trained and how exactly the test samples are excluded from scoring.
- [References] Shaham et al. (2024a) and (2024b) appear to be the same paper and should be cited once. The reference 'Zeping and Sophia (2024)' is non-archival ('Under review'); since it motivates the attention-only design, a citable version or additional evidence would be helpful.
- [Table 1 caption] The caption 'evaluated against their SFT baselines' is ambiguous; clarify that the win rate is computed against the corresponding SFT-trained model.
Circularity Check
The core loss reduces to standard DPO on cross-lingual pairs, and the main AlpacaEval metric rewards the exact behavior CLO is trained to enforce; external benchmarks temper the circularity.
-
renaming known result
[Appendix A, Eqs. (10)-(12); Section 3.2]
"In the standard DPO approach with cross-lingual data augmentation, the loss function is applied independently to each language's data, and the model does not utilize cross-lingual data pairs within the same batch. ... This is fundamentally different from just applying DPO on cross-lingual data augmentation."
The claimed distinction is contradicted by the paper's own equations. L_CL in Eq. (10) is an average of log sigma(z_en^{(i)}) + log sigma(z_l^{(i)}), where z_en^{(i)} is the DPO log-ratio for the pair (x_en, y_en, y_l) and z_l^{(i)} is the DPO log-ratio for (x_l, y_l, y_en). Averaging these terms is exactly the standard DPO loss on the combined cross-lingual augmented dataset; there is no coupling between different samples i or any joint batch-level term. Thus the 'cross-lingual loss' is a relabeling of standard DPO applied to translated preference pairs, not a new objective that 'differs from the standard DPO loss.'
-
other
[Appendix C (AlpacaEval setup); Section 3.2, Eq. (7)]
"Additionally, since the purpose is to measure multilinguality, if a model responds in a language different from the instruction's language, this should be reflected in the evaluation."
The CLO loss in Eq. (7) is specifically designed to increase log pi(y_l|x_l) and decrease log pi(y_en|x_l), i.e. to suppress English responses to target-language inputs. The modified AlpacaEval evaluator is explicitly instructed to penalize responses in a language different from the instruction. Therefore the target-language win-rate advantage reported in Table 1 is partly enforced by aligning the metric with the training objective, rather than measuring transferred knowledge independently. The BELEBELE and MMMLU results are external and not circular, which limits the severity.
full rationale
The paper does not fit parameters to its target benchmarks, and it does not rely on a load-bearing self-citation chain or an imported uniqueness theorem. The genuinely circular component is the claimed novelty of the loss: L_CL in Eq. (10) is, up to a constant, the standard DPO loss on the two cross-lingual preference pairs (x_en, y_en, y_l) and (x_l, y_l, y_en). The Appendix A claim that CLO is 'fundamentally different' from standard DPO on cross-lingual data is not supported by the equations, since each sample's z_en and z_l are independent DPO log-ratios and the average over the batch is just DPO on the augmented dataset. A second, partial issue is that the main AlpacaEval metric was modified to penalize language mismatch, which is exactly the behavior CLO's Eq. (7) is trained to produce; this makes part of the headline win-rate and data-efficiency results on AlpacaEval favorable by construction. However, the paper is not purely circular: all models are also evaluated on BELEBELE and MMMLU, which do not encode the CLO objective, and those external benchmarks independently show gains. The overall circularity is therefore partial and concentrated in the loss-formulation claim and the AlpacaEval evaluation design, not in every reported result.
Assumptions & free parameters
free parameters (2)
- λ (trade-off parameter) =
0.5
- β (DPO KL coefficient) =
0.1
assumptions (4)
- domain assumption The model's internal knowledge is language-independent and can be expressed in the target language once output-language bias is corrected.
- domain assumption Translated target-language data (via M2M100) preserves the instruction-response semantic content well enough for preference learning.
- domain assumption Training only attention layers is sufficient for cross-lingual transfer.
- domain assumption The GPT-4o evaluation with language-specific prompts and language-mismatch penalty is a valid measure of target-language instruction following.
Cite this review
Pith. "Pith review of Cross-Lingual Optimization for Language Transfer in Large Language Models." pith.science (2026). https://pith.science/paper/MEWYLTWH
@misc{pith2026250514297,
author = {Pith},
title = {Pith review of: Cross-Lingual Optimization for Language Transfer in Large Language Models},
year = {2026},
howpublished = {\url{https://pith.science/paper/MEWYLTWH}},
note = {Machine review of arXiv:2505.14297}
}
read the original abstract
Adapting large language models to other languages typically employs supervised fine-tuning (SFT) as a standard approach. However, it often suffers from an overemphasis on English performance, a phenomenon that is especially pronounced in data-constrained environments. To overcome these challenges, we propose \textbf{Cross-Lingual Optimization (CLO)} that efficiently transfers an English-centric LLM to a target language while preserving its English capabilities. CLO utilizes publicly available English SFT data and a translation model to enable cross-lingual transfer. We conduct experiments using five models on six languages, each possessing varying levels of resource. Our results show that CLO consistently outperforms SFT in both acquiring target language proficiency and maintaining English performance. Remarkably, in low-resource languages, CLO with only 3,200 samples surpasses SFT with 6,400 samples, demonstrating that CLO can achieve better performance with less data. Furthermore, we find that SFT is particularly sensitive to data quantity in medium and low-resource languages, whereas CLO remains robust. Our comprehensive analysis emphasizes the limitations of SFT and incorporates additional training strategies in CLO to enhance efficiency.
Figures
Figures from the paper (2 more)
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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