Challenge: Autoregressive sequence-to-sequence (seq2sequ) neural architectures have become the de facto approach in Machine Translation (MT).
Approach: They propose to make models less reliant on cross-entropy loss and evaluation metrics . they propose an entropicity-regularised RL method that explores the action space .
Outcome: The proposed method exploits the action space and unsupervised reward function to balance between exploration and exploitation.

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Breaking Consensus Bias: Unsupervised Reinforcement Learning for Machine Translation (2026.findings-acl)

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Challenge: Existing RL approaches for MT face fixed references or the production of homogeneous references leading to mode collapse in unsupervised settings.
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SURF: Semantic-level Unsupervised Reward Function for Machine Translation (2022.naacl-main)

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Challenge: Reinforcement Learning (RL) is dependent on the reward formulation due to the intrinsic difficulty of the task in the high-dimensional discrete action space and the sparseness of the standard reward functions.
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MT-R1-Zero: Advancing LLM-based Machine Translation via R1-Zero-like Reinforcement Learning (2025.findings-emnlp)

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Challenge: Large-scale reinforcement learning (RL) methods have proven effective in enhancing the reasoning abilities of large language models.
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Fine-Grained Reward Optimization for Machine Translation using Error Severity Mappings (2026.tacl-1)

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Challenge: Reinforcement learning (RL) is an effective and robust method for training neural machine translation systems.
Approach: They propose a method that leverages fine-grained, token-level quality assessments . they use a state-of-the-art quality estimation system as their token- level reward model .
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Enhancing Reinforcement Learning with Dense Rewards from Language Model Critic (2024.emnlp-main)

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Challenge: Reinforcement learning (RL) can align language models with non-differentiable reward signals, such as human preferences, but the sparsity of these signals can lead to inefficient and unstable learning.
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Countering Reward Over-Optimization in LLM with Demonstration-Guided Reinforcement Learning (2024.findings-acl)

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Challenge: Existing approaches address ROO by adding KL regularization, requiring computationally expensive hyperparameter tuning.
Approach: They propose a reinforcement learning approach that leverages human demonstrations and a reward model to recalibrate the reward objective.
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A Study of Reinforcement Learning for Neural Machine Translation (D18-1)

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Challenge: Recent studies have shown that reinforcement learning (RL) is an effective approach for improving the performance of neural machine translation systems.
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RED: Unleashing Token-Level Rewards from Holistic Feedback via Reward Redistribution (2025.emnlp-main)

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Challenge: Experimental results demonstrate the superiority of our approach to aligning large language models with human preferences.
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Reinforcement Learning with Large Action Spaces for Neural Machine Translation (2022.coling-1)

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Challenge: Recent work has argued that the gains produced by Reinforcement learning are mostly due to promoting tokens that have already received a fairly high probability in pre-training.
Approach: They hypothesize that the large action space is a main obstacle to RL’s effectiveness in MT by reducing the size of the vocabulary without changing the vocabulary.
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RIVAL: Reinforcement Learning with Iterative and Adversarial Optimization for Machine Translation (2025.findings-emnlp)

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Challenge: Using reinforcement learning from human feedback, large language models perform poorly when applied to colloquial subtitle translation tasks.
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