The Alignment Gap: A Benchmark Demonstrating the Lack of Cross-Lingual Mapping in Dialect-Specialized Language Models - The Case of Ehugbo
Cross-lingual information retrieval (CLIR) for low-resource dialects remains underexplored, despite millions of speakers worldwide. This work addresses a critical gap by introducing the first information retrieval (IR) benchmark resource for Ehugbo (the Afikpo dialect of Igbo with \textasciitilde 150,000 speakers in Nigeria), constructed from a high-quality parallel multimodal corpus: 1 hour of transcribed Ehugbo Bible audio aligned with standard English translations. This parallel design enables rigorous evaluation of retrieval models across language pairs. Our benchmark reveals a surprising and counterintuitive finding: the ''Alignment Gap'', where African-centric foundation models (Serengeti, Afro-XLMR, AfriBERTA) that excel at linguistic familiarity with Igbo achieve <5% retrieval accuracy, while global models like LaBSE achieve 85% despite less exposure to the language family. Through diagnostic analysis (t-SNE visualizations, tokenization studies, error patterns), we show that regional models lack cross-lingual alignment bridges despite deep language understanding, while global models achieve language invariance through explicit translation supervision. This finding has immediate implications for the design of multilingual systems: pre-training diversity alone is insufficient for dialectal IR. We release our Ehugbo corpus, results, and evaluation splits on GitHub to enable future work on dialect-specific fine-tuning and alignment strategies for African languages.