There's a patch (to mypaint rather than libmypaint) to resolve the conflict without a new version of mypaint, by renaming mypaint's locale to 'mypaint12', in the upstream tracker at <https://github.com/mypaint/mypaint/issues/918#issuecomment-433645267>.
This is the approach previously suggested by Chris Waters in <https://bugs.debian.org/894757#25>. I haven't tested it myself, but it looks plausible. (Cc'd the existing mypaint bug for this conflict, <https://bugs.debian.org/897909>.)
