Sendy now includes translation support. You can safely translate the whole of Sendy without worrying about future updates overwriting them. Translating is also easy, just follow this guide http://sendy.co/translation
If you're going to translate Sendy into a particular language, you can do so at anytime. Or, check the translation category in this forum to see if anyone has started translation into your language. If so, I encourage you to reply the user's thread to request to "co-translate" Sendy. You can even PM each other or request for each other's email to communicate and send files. This will lead to a higher quality translation.
If no one has started translating into your language and you have translated it yourself, consider starting a new thread with a download link to your translation file to share it with others.
Let me know if you have any questions!