Plug in

Hey! I ran into something like this recently and what worked for me was using an AE tool called Cult Translator. It handles the translations inside After Effects automatically, so I didn’t have to copy/paste everything manually.
youtu.be/6UKt2QWfaws 

@Mathias Moehl iis correct — the plugins won't do the actual translations directly within AE.  You can accomplish that using a different toolset like Google Sheets using the =GOOGLETRANSLATE() function, and then bring that data into AE.

There are also caveats to setting up your text layers regarding different languages that you will want to consider regarding fonts and character sets.  Check out this podcast where I review how to prepare your AE text layers for different languages:  https://www.youtube.com/watch?v=GA_H2shE_AY

I am not aware of an extension, which does the actual translations for you.

But with my extension Text2Spreadsheet, you can export all texts of the Ae project to a spreadsheet. Then you can import the spreadsheet into google sheets and translate the texts there.

Finally, you export the translated spreadsheet again into a csv file and import it with Text2Spreadsheet back to After Effects.