Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages?

Donghoon Han, SungHyun Moon, Aidyn Zhakatayev, Junghun Cha, SeungJae Lee


Abstract
Recent multilingual vision–language encoders cover hundreds of languages in a single model, yet on two state-of-the-art instances retrieval on low-resource languages (LRL; e.g. Swahili) trails high-resource ones (HRL; e.g. English) by 30+ pp. We ask where in the trained encoder this gap is located. Prior modality-gap and cross-lingual subspace work suggests a linear language direction at the output crowds out alignment-relevant geometry. We falsify this: LEACE drives the linear language classifier from >99% to near chance and iterated INLP to 37–50% while LRL retrieval moves within ± 1.5 pp and all tier means within 2.2 pp, tracking random controls. The linear bias is a symptom, not the cause. Instead, the alignment-causal factor lies along the encoder’s forward path: the EOS (end-of-sequence) hidden state’s per-language trajectory diverges with depth. Substituting the EOS with its parallel English value three blocks before the projector lifts Swahili from 22.1% to 69.1% on one encoder (and reproduces on the other); three controls rule out pooled-position tautology and English specificity. A front-layer trunk that pulls each language’s projection toward the parallel-content centroid corroborates the diagnosis at training time, recovering +9.6 / +17.1 pp on LRL XM3600 retrieval (1,000-image subset), with consistent gains across three further benchmarks while preserving HRL performance.
Anthology ID:
2026.emnlp-main.840
Volume:
Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
EMNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
19474–19505
Language:
URL:
https://aclanthology.org/2026.emnlp-main.840/
DOI:
Bibkey:
Cite (ACL):
Donghoon Han, SungHyun Moon, Aidyn Zhakatayev, Junghun Cha, and SeungJae Lee. 2026. Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages?. In Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing, pages 19474–19505, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages? (Han et al., EMNLP 2026)
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PDF:
https://aclanthology.org/2026.emnlp-main.840.pdf