Directly editing the language files is not recommended. These files may be overwritten during module updates, resulting in a loss of any edits you had made.
Instead, it is recommended to use overrides. Overrides allow you to customise the default variables with your own values such that they will be preserved through the update process.
To create an override, copy the language file into the directory /modules/addons/addondomain/lang/override/ and edit the text you want to change.