MATE 1.14 will be released soon (no precise date, but probably by this weekend or next week) as upstream are now busy with tarballs generation and doing final testing before launching it for public.
Since Slackware 14.2 is now at RC 1, my plan is to keep building MATE 1.12 for Slackware 14.2 users first and then i can move my focus on preparing MATE 1.14 for Slackware 14.2. MATE 1.12 is very stable at this moment and upstream still keep providing bug fixes to this version, so it's very suitable for Slackware 14.2.
With MATE 1.14, some MATE sub components are now GTK+3 build only as can be seen on the ROADMAP. More components will be ported to GTK+3 build only in future releases. This means that for MATE 1.14, i may provide a mix build of GTK+2 and GTK+3. It's less ideal actually and i'm looking for a possibility of switching to full GTK+3 only since i already have a separate branch for GTK+3 build only. We will see about that when i started to play with MATE 1.14.
Meanwhile, you can help me to decide by giving your votes about how i should build MATE 1.14 for Slackware 14.2. Please vote :)