mousepad has plugin support since 0.5.5. The only plugin available currently is Spell Checking plugin. For someone like me who uses mousepad to do pretty much every type of writing, this Spell Checking plugin is very handy. Sadly, this plugin depends on gspell and gspell is not available in the default Slackware system. So when Pat or whoever compiles default mousepad package for Slackware, it gets build without the Spell Checking plugin. Compiling/Installing gspell is pretty simple and straight forward as all of its dependencies are available by default in standard Slackware. So my request/recommendation to gapan and Salix team would be to make sure they build mousepad with Spell Checking plugin support unlike Slackware.
P.S. The Spell Checking plugin languages can be extended by aspell and its various dictionaries.
https://docs.xfce.org/apps/mousepad/start
https://gitlab.gnome.org/GNOME/gspell
http://aspell.net/