Template overrides are for layout files. You can override a plugin's layout/template files, assuming the plugin is designed this way. But this cannot be used to change behavior of system plugins. Usually you have to write your own plugin to use that instead of the one you want to override.
In your case you could copy the Language Filter Plugin, rename it wherever it needs renaming and then adjust the code where you want it. Then when your plugin is ready, disable the core one and use yours in its place.
Note that you will have to maintain and update your plugin when needed in order to keep it compatible with newer Joomla versions.
Follow the resources here to find out more about Plugin Development:
Documentation: Plugin Development
By the way:
What type of changes you want to do on the language filter plugin? What is not working the way you want and what you are trying to achieve?