How to merge custom audio stream in .mkv file?

Need help.... here is the situation
I have two movies of harry potter philosophers stone, one in high definition blu-ray format .mkv format (12GB and the second one in bad quality (999MB, audio is of good quality but is in the language i want), i want to copy the audio stream from the second one and merge the audio stream into the first one,the blu-ray HDVD file, perfectly so that it is well synchronised with the video, please tell me the software to do this?