Effects of Pivot Prompting and Text Type on LLM Translation Quality for the Low-Resource Chinese–Vietnamese Pair: Evidence from COMET and Human Evaluation

Large language models (LLMs) such as ChatGPT have advanced machine translation, but their quality on low-resource language pairs remains uneven and is typically assessed with automatic metrics alone. This study examines how prompting strategy and source-text type jointly affect LLM translation quality for the low-resource Chinese–Vietnamese pair and whether automatic and human assessments agree. Using a 2 × 3 mixed factorial design, we compared an English-mediated pivot strategy with a direct strategy across informative, expressive, and operative texts, evaluating 60 ChatGPT translations with COMET and with 15 bilingual readers who rated adequacy, fluency, faithfulness, trustworthiness, willingness to use, and need for revision. On COMET, pivoting significantly improved overall quality (p < 0.001), text type was the dominant factor (η2 = 0.91; informative > operative > expressive), and strategy interacted with text type, with the largest pivot gain for expressive texts. Human ratings reproduced this ordering but diverged sharply for expressive texts: although COMET favoured the pivot output, readers reported that pivoting raised fluency yet substantially reduced faithfulness (4.10 → 1.77 on a 7-point scale) and were unwilling to accept it. These results show that the value of a prompting strategy is text-type-dependent and that fluent LLM output is not necessarily faithful, underscoring the need to pair automatic metrics with human evaluation when benchmarking low-resource translation.