I’ve been fighting this for awhile as the mkvtoolnix package doesn’t have the correct dependencies when you install it. Well, after doing a bit more digging I’ve found the answer to get mkvmerge to work!
Then, you need to set an environment variable of LANG=C. This can be done from the command line.
*UPDATE – Jan 1 2016*
In addition to the LANG=C environment variable being set, I’ve also had to add in LC_ALL=C. For me, I run a script that calls mkvmerge, so I just set both of these in the script.