If you have access to the plugin directory I assume you could do the follow:
Go into directory nodebb-plugin-contact-page/languages/
Make a copy of the directory en-GB and name it he
Go into the new directory he and open file contactpage.json
Change english texts on the right to the corresponding hebrew texts and save.
You may need to rebuild and restart nodebb.
I don't see any other way to change it, but if you can't make these changes then you'll likely need to request them from the plugin developer.