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Building a One-Million-Pair Bokmål–Nynorsk Translation Corpus: A Quality-First Harvesting and Cleaning Pipeline

We present a high-quality parallel corpus for translation between Norwegian Bokmål (nb) and Nynorsk (nn), two closely related written standards of Norwegian. The corpus was assembled from two complementary sources: Nasjonal digital læringsarena (NDLA), an educational platform, and Nynorsk pressekontor (NPK), a newswire service. Our methodology prioritizes precision over volume, employing a multi-stage filtering pipeline designed to address the specific challenges of aligning near-neighbor languages. This pipeline combines paragraph-level alignment, deduplication, multilingual semantic similarity scoring, language identification confidence checks, structural consistency tests, and strict bidirectional adjudication by a Large Language Model (LLM). To address the common problem of untranslated or placeholder "pending" copies, we apply a rule that flags pairs with zero semantic distance when the Nynorsk side shows weak evidence of being distinctively Nynorsk. After filtering, we retained 191,695 pairs from NDLA and 809,164 pairs from NPK, resulting in a merged corpus of 1,000,859 parallel paragraphs. This resource demonstrates that a precision-oriented pipeline can produce data better suited for training robust machine translation systems and instruction-tuned models than larger but noisier alternatives.