I get the same error whenever I try to install software on my laptop, for example: I want to install Pavucontrol. So, I open the terminal and I type sudo apt-get install pavucontrol and my terminal output is:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  libgconfmm-2.6-1c2 libglademm-2.4-1c2a libpulse-mainloop-glib0 padevchooser
  paman paprefs pavumeter pulseaudio-module-zeroconf
The following NEW packages will be installed:
  libgconfmm-2.6-1c2 libglademm-2.4-1c2a libpulse-mainloop-glib0 padevchooser
  paman paprefs pavucontrol pavumeter pulseaudio-module-zeroconf
0 upgraded, 9 newly installed, 0 to remove and 172 not upgraded.
1 not fully installed or removed.
Need to get 0B/345kB of archives.
After this operation, 2044kB of additional disk space will be used.
Do you want to continue [Y/n]? Y
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LANGUAGE = (unset),
    LC_ALL = (unset),
    LANG = "el_GR.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
Can't exec "locale": No such file or directory at /usr/share/perl5/Debconf/Encoding.pm line 16.
Use of uninitialized value $Debconf::Encoding::charmap in scalar chomp at /usr/share/perl5/Debconf/Encoding.pm line 17.
dpkg: `ldconfig' not found on PATH.
dpkg: 1 expected program(s) not found on PATH.
NB: root's PATH should usually contain /usr/local/sbin, /usr/sbin and /sbin.
E: Sub-process /usr/bin/dpkg returned an error code (2)

What is the problem and how do I fix it?