Not open source, but https://fluen.ai https://fluen.ai does a good job at translating subs, while using your own or standards style guides for the target langua

Not open source, but https://fluen.ai https://fluen.ai does a good job at translating subs, while using your own or standards style guides for the target language - ie. reading speed, max characters, "chunking" sentences where it grammatically makes sense, re-adapting them etc.