What to do depends on details about your use-case.
Basically you're always free to fork and edit your own copy of any plugin (as long you meet the matters of its license(s)).
So yes, you can overwrite existing localisations. Whether it's easy depends on the plugin. As long this already features a structure like @yariplus has shown above, it's pretty easy to add additional languages or replace some translations.
But if it written without featuring NodeBBs i18n-system, you'd have to edit the code-base directly (in this case I can recommend the docs).
Basically if the plugin.json already contains a
languages key, you're most likely having an easy time editing/enhancing those.
And of course it's always nice to create pull-requests for the original plugin to merge your changes since more language support is most often a task for the community rather than the plugin author itself.
@yariplus Wouldn't you need