The Benefits of Being Uncertain: Perplexity as a Signal for Naturalness in Multilingual Machine Translation

AbstractModel-internal uncertainty metrics like perplexity potentially offer low-cost signals for Machine Translation Quality Estimation (TQE). This paper analyses perplexity in the No Language Left Behind (NLLB) multilingual model. We quantify a significant model-human perplexity gap, where the model is consistently more confident in its own, often literal, machine-generated translation than in diverse, high-quality human versions. We then demonstrate that the utility of perplexity as a TQE signal is highly context-dependent, being strongest for low-resource pairs. Finally, we present an illustrative case study where a flawed translation is refined by providing potentially useful information in a targeted prompt, simulating a knowledge-based repair. We show that as the translation’s quality and naturalness improve (a +0.15 COMET score increase), its perplexity also increases, challenging the simple assumption that lower perplexity indicates higher quality and motivating a more nuanced view of uncertainty as signalling a text’s departure from rigid translationese.