There are two different translation keys used for the reply button. js.composer.reply is used for the reply button that you see when you open the composer. js.topic.reply.title is used for the reply button is displayed in the topic (the button that you click to open the composer.)
I have not memorized these translation keys. The way I found them was by entering I18n.enableVerboseLocalizationSession() into the console of my browser’s web inspector. After doing that, a number key will be added to all text items on Discourse that can be customized. You can use that number to find the correct translation key. That approach is outlined here: Find a translation key with verbose localization.
On the latest Chrome browser, the translation key names are displayed in the web inspector’s ‘info’ tab.