It seems that the sk.po localization has not been included in the xfce4-mount-plugin-1.1.2.tar.gz release archive. Nevertheless, 'sk' is listed in XDT_I18N in configure.ac. As a result, attempting to install xfce4-mount-plugin with LINGUAS containing sk fails:

$ LINGUAS=sk make install DESTDIR=/tmp/z install 
Making install in panel-plugin
make[1]: Entering directory '/tmp/xfce4-mount-plugin-1.1.2/panel-plugin'
make[2]: Entering directory '/tmp/xfce4-mount-plugin-1.1.2/panel-plugin'
make[2]: Nothing to be done for 'install-exec-am'.
 /bin/mkdir -p '/tmp/z/usr/local/share/xfce4/panel/plugins'
 /usr/bin/install -c -m 644 xfce4-mount-plugin.desktop '/tmp/z/usr/local/share/xfce4/panel/plugins'
 /bin/mkdir -p '/tmp/z/usr/local/lib/xfce4/panel/plugins'
 /bin/sh ../libtool   --mode=install /usr/bin/install -c   libmount.la '/tmp/z/usr/local/lib/xfce4/panel/plugins'
libtool: install: /usr/bin/install -c .libs/libmount.so /tmp/z/usr/local/lib/xfce4/panel/plugins/libmount.so
libtool: install: /usr/bin/install -c .libs/libmount.lai /tmp/z/usr/local/lib/xfce4/panel/plugins/libmount.la
libtool: warning: remember to run 'libtool --finish /usr/local/lib/xfce4/panel/plugins'
make[2]: Leaving directory '/tmp/xfce4-mount-plugin-1.1.2/panel-plugin'
make[1]: Leaving directory '/tmp/xfce4-mount-plugin-1.1.2/panel-plugin'
Making install in po
make[1]: Entering directory '/tmp/xfce4-mount-plugin-1.1.2/po'
make[1]: *** No rule to make target 'sk.gmo', needed by 'all-yes'.  Stop.
make[1]: Leaving directory '/tmp/xfce4-mount-plugin-1.1.2/po'
make: *** [Makefile:457: install-recursive] Error 1