sudo apt full-upgrade
Reading package lists… Done
Building dependency tree
Reading state information… Done
Calculating upgrade… Done
The following package was automatically installed and is no longer required:
libpoppler57
Use ‘sudo apt autoremove’ to remove it.
The following packages will be REMOVED:
language-selector-gnome ubuntu-mate-core
Maybe you should start using Synaptic Package Manager. It can help with such questions 
About meta packages
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Because your also trying to remove gnome-language-selector which is required by ubuntu-mate-core