When trying to edit and save preferences, makemkv crashes upon pressing the "OK" or "Apply" button of the dialog (Segmentation fault). Can be reproduced on x86 (update of self built 1.7.8) as well as on amd64 (fresh installation). I will provide further details (stack trace) later.
My fault, i should have read the post installation message regarding default.mmcp.xml. However: when i compile makemkv by myself, this file is neither existent nor necessary, although my installation location is non-standard as well (/usr/local/something). And: with or without this file the localisation does not work. Although the .mo files are installed (even in /usr/share), they don't seem to be found; the LANG environment is ignored and there are no other interface languages offered for manual selection than english. There should be german, spanish, japanese, and others.
I haven't been able to confirm the localization issues you've mentioned before (it works perfectly for me, but I'm en_US based), but the save preferences gui can be a little temperamental....there are some options that when set seem to confuse the system, so one has to do some trial and error to see what one can modify. It took me about 10 minutes to figure out the right defaults when I set it up here... You may want to discuss your concerns directly with upstream as they have a user forum.
Please try again with 1.8.7
Please test against 1.8.11 which was just pushed to the tree, if there is no confirmation that this issue still exists, I will close bug within the next month.