By installing additional package with plugins translations (if available), changing to other translation extension, or by translating extensions on your own. Translation files path is: FORUMDIRECTORY/vendor/AUTHOR/ExtensionTITLE/resources/locale/en.yml
You can copy those files to your computer and translate, then replace. Be aware though, that they will disappear with such plugin's update you will ever download.
The best solution would be for you to create your own extension providing extension's translations, so that they won't disappear with updates, or to copy extensions translations to the translation package you are already using, if you really don't want to go with the first option.
It isn't ideal solution, but it has better chance to not be resetted - as far as you won't update translation package, it will do.
To do so, you need to rename downloaded extension's translations from en.yml to author-title - for example: fof-merge-discussions, and after changing them to your satisfaction, upload to FORUMDIRECTORY/vendor/YourTranslationPackageAUTHOR/TranslationPackageTITLE/locale.
Then you may want to re-enable the extension.
