your snap tries to access /usr/share/locale/ which is denied … i fear you have to teach it to not do that.
if your app respects locale standards [1] you should be able to set LOCPATH and I18NPATH in the environment (from a wrapper script around “kf5-launch konversation”) to point to a dir the app can actually access (looking at your yaml i assume thats $SNAP/kf5/usr/share/locale )
are locales usually shipped in kde-frameworks-5 ? if so, make sure they are accessible in the shared directory and point the above variables to the right places.
under [2] is also a more detailed description of the content interface.
[1] http://manpages.ubuntu.com/manpages/zesty/man1/locale.1.html
[2] The content interface