One Model to Translate Them All? A Journey to Mount Doom for Multilingual Model Merging

Title:One Model to Translate Them All? A Journey to Mount Doom for Multilingual Model Merging

Abstract:Weight-space model merging combines independently fine-tuned checkpoints without access to the original training data. While merging has shown promise in multitask settings, its behavior in multilingual generative systems remains underexplored. We systematically study weight-space merging for multilingual machine translation by fully fine-tuning language models on large-scale bilingual corpora and evaluating representative merging strategies across shared-source, shared-target, and bidirectional consolidation settings. Our experiments reveal a strong directional asymmetry. Merging is comparatively more effective when models share a target language, improving multilingual coverage over the base model, but it still fails to preserve the peak performance of language-specific checkpoints. In contrast, when target languages differ, performance degrades sharply, especially in shared-source and bidirectional settings. To explain this behavior, we analyze internal representations and find that fine-tuning does not create disjoint language-specific sub-networks. Instead, independently fine-tuned models activate largely overlapping neurons while reshaping upper-layer target-generation representations into incompatible geometries. These findings suggest that multilingual merging failures arise from target-side geometric misalignment within shared computational units, challenging the assumptions underlying standard weight-space merging for multilingual translation. We make the code publicly available at this https URL

Computation and Language (cs.CL); Artificial Intelligence (cs.AI)

arXiv:2604.02881 [cs.CL]

(or arXiv:2604.02881v2 [cs.CL] for this version)

https://doi.org/10.48550/arXiv.2604.02881 Focus to learn more arXiv-issued DOI via DataCite

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