@inproceedings{han-etal-2026-multilingual,
title = "Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages?",
author = "Han, Donghoon and
Moon, SungHyun and
Zhakatayev, Aidyn and
Cha, Junghun and
Lee, SeungJae",
editor = "Lee, Hung-Yi and
Dev, Sunipa and
Li, Jessy and
Elliott, Desmond",
booktitle = "Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing",
month = oct,
year = "2026",
address = "Budapest, Hungary",
publisher = "Association for Computational Linguistics",
url = "https://aclanthology.org/2026.emnlp-main.840/",
pages = "19474--19505",
ISBN = "979-8-89176-459-0",
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 $\pm 1.5$ 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."
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<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 \pm 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.</abstract>
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%0 Conference Proceedings
%T Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages?
%A Han, Donghoon
%A Moon, SungHyun
%A Zhakatayev, Aidyn
%A Cha, Junghun
%A Lee, SeungJae
%Y Lee, Hung-Yi
%Y Dev, Sunipa
%Y Li, Jessy
%Y Elliott, Desmond
%S Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
%D 2026
%8 October
%I Association for Computational Linguistics
%C Budapest, Hungary
%@ 979-8-89176-459-0
%F han-etal-2026-multilingual
%X 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 \pm 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.
%U https://aclanthology.org/2026.emnlp-main.840/
%P 19474-19505
Markdown (Informal)
[Where Do Multilingual Vision-Language Encoders Fail on Low-Resource Languages?](https://aclanthology.org/2026.emnlp-main.840/) (Han et al., EMNLP 2026)
ACL