Reinforcement learning (RL) has shown strong promise for LLM-based machine translation, with recent methods such as GRPO demonstrating notable gains; nevertheless, translation-oriented RL remains challenged by high-variance policy gradients induced by Monte Carlo baselines, as well as a large trajectory space that favors global exploration over fine-grained local optimization. We introduce PEGRL, a two-stage RL framework that uses post-editing as an auxiliary task to stabilize training and guide overall optimization. At each step, translation outputs are sampled to construct post-editing inputs, enabling lower-variance gradients from the post-editing task to propagate through the entire framework while jointly supporting both global exploration and fine-grained local optimization. A task-specific weighting scheme further emphasizes the post-editing gradient, producing a biased yet more sample-efficient estimator. Experiments on English→Finnish, English→Turkish, and English↔Chinese show consistent gains over RL baselines, and for English→Turkish, performance on COMETKiwi is comparable to advanced LLM-based systems (DeepSeek-V3.2). Our code and a set of representative pretrained models are publicly available at https://github.com/NJUNLP/peg-rl and https://huggingface.co/collections/DGME/pegrl.

Expanding Large Language Models(LLMs) to new languages is a costly endeavor, demanding extensive Continued Pre-Training(CPT) and data-intensive alignment. While recent data-free merging techniques attempt to bypass alignment by fusing a multilingual CPT-enhanced model with its instruct counterpart, they are plagued by a critical trade-off: mitigating parameter conflicts to preserve original abilities inevitably dilutes new language acquisition, and vice-versa. To resolve this conflict, we introduce , which upcycles a dense model into a Mixture-of-Experts(MoE) architecture, allocating different experts to different languages. Alignment ability is then transferred by grafting a MoE-expanded parameter delta(𝛥_instruct) to the CPT-enhanced base model, bypassing the complex alignment phase. Experiments demonstrate ’s superiority even against baselines with similar FLOPs or number of parameters; it improves performance on expanded languages while effectively preserving original capabilities. We further show our approach is highly applicable across different models and Post-training deltas.

Large language models (LLMs) exhibit remarkable multilingual capabilities despite the extreme language imbalance in the pre-training data. In this paper, we closely examine the reasons behind this phenomenon, focusing on the pre-training corpus. We find that the existence of code-switching, alternating between different languages within a context, is key to multilingual capabilities. We conduct an analysis to investigate code-switching in the pre-training corpus, examining its presence and categorizing it into four types within two quadrants. We then assess its impact on multilingual performance. These types of code-switching data are unbalanced in proportions and demonstrate different effects on facilitating language transfer. To better explore the power of code-switching for language alignment during pre-training, we investigate the strategy of synthetic code-switching. We continuously scale up the synthetic code-switching data and observe remarkable improvements in both benchmarks and representation space. Extensive experiments indicate that incorporating synthetic code-switching data enables better language alignment and generalizes well to high, medium, and low-resource languages with pre-training corpora of varying qualities.

Large-scale pretrained language models (LLMs), such as ChatGPT and GPT4, have shown strong abilities in multilingual translation, without being explicitly trained on parallel corpora. It is intriguing how the LLMs obtain their ability to carry out translation instructions for different languages. In this paper, we present a detailed analysis by finetuning a multilingual pretrained language model, XGLM-7.5B, to perform multilingual translation following given instructions. Firstly, we show that multilingual LLMs have stronger translation abilities than previously demonstrated. For a certain language, the translation performance depends on its similarity to English and the amount of data used in the pretraining phase. Secondly, we find that LLMs’ ability to carry out translation instructions relies on the understanding of translation instructions and the alignment among different languages. With multilingual finetuning with translation instructions, LLMs could learn to perform the translation task well even for those language pairs unseen during the instruction tuning phase.

“机器翻译旨在通过计算机自动将一种自然语言翻译成另一种自然语言,这个过程对于机器翻译模型的语言理解、语言生成能力有着极高的要求。因此机器翻译一直以来都是一项极具研究价值和研究难度的自然语言处理任务。近期研究表明,大语言模型能够根据人类指令完成包括翻译在内的许多任务,在这一过程中展现出强大的语言理解和生成能力,为自然语言处理范式革新提供了新的可能。为了在大语言模型支持下更好地完成机器翻译任务,研究人员对大语言模型的机器翻译和多语言能力进行了大量的研究和分析。本文从以下三方面介绍相关研究热点和最新进展,包括:大语言模型翻译能力评估、大语言模型翻译能力激发、大语言模型在不同语言上的能力展现。”