MCAT: Scaling Many-to-Many Speech-to-Text Translation with MLLMs to 70 Languages

Multimodal Large Language Models (MLLMs) have achieved great success in Speech-to-Text Translation (S2TT) tasks. However, current research is constrained by two key challenges: language coverage and efficiency. Most of the popular S2TT datasets are substantially English-centric, which restricts the scaling-up of MLLMs’ many-to-many translation capabilities. Moreover, the inference speed of MLLMs degrades dramatically when the speech is converted into long sequences (e.g., 750 tokens). To address these limitations, we propose a <underline>M</underline>ultilingual <underline>C</underline>ost-effective <underline>A</underline>ccelerated Speech-to-Text <underline>T</underline>ranslator (MCAT) framework, which includes two innovations. First, a language scaling method that leverages curriculum learning and a data balancing strategy are introduced to extend the language coverage supported by MLLMs to 70 languages and achieve mutual translation among these languages. Second, an optimized speech adapter module is designed to reduce the length of the speech sequence to only 30 tokens. Extensive experiments were conducted on MLLMs of different scales (9B and 27B). The experimental results demonstrate that MCAT not only surpasses state-of-the-art end-to-end models on the FLEURS dataset across <inline-formula><tex-math notation="LaTeX">$\mathbf {70 \times 69}$</tex-math></inline-formula> directions but also enhances batch inference efficiency. This is achieved with only <inline-formula><tex-math notation="LaTeX">$ \mathbf {\sim 100M}$</tex-math></inline-formula> trainable parameters and by using only 10 hours of S2TT data per language. Furthermore, we have released MCAT as open-source to promote the development of MLLMs for robust S2TT capabilities.