Everything that can be found in a language file should be translatable from Linguist.
However you need to have at least one language with the translations enabled. So if an extension is only translated in English, you need to enable English in Flarum at least for the time to make the translations. Once you have translated everything, you can disable English again.
Another option is to use the new Import feature in Linguist, and import the en.yml
file as another language, then edit it through Linguist. (Or edit before importing, that works too)